A video I watched said that you should always use parametric modelling when drawing sketches. So do you always have to use it or do you only use it in certain scenarios. Greatly appreciate any answers 
Well, sketches are there to create complex surfaces from measurements. Of course you can just add some squares and circles and give them more or less sizes to get a quick result.
But if you want a bigger version later on or a change, then this results in a lot of work as you need to resize, realign and recenter stuff. Just take your dog tag for example. If you want a smaller version you need to change sizes for the text and the hole, too. While a fully parametric model just takes a new outer radius and done. (involving FreeCAD text as an example here is a bad idea, but you get what I mean)
Even for simple things this is a good idea. Better to it right once, than waste time over and over later on.
Since a spreadsheet is a good way to start anyway to get real world measurements into the CAD application, it saves time when drawing them and prevents errors as you cannot make a typo. In the end model sizes react on the spreadsheet as a benefit from that.
Naming sketches and operations will also help to get into old models later on. Basically the same rules apply for software development. No numbers in documents and long labels.
It really doesn’t take much longer to create a model parametrically. Hardly seems worth not doing it.
The short answer is no. You do not have to replace your dimensions with parameters but it is best practice. Overall for me, if I am doing a throwaway model, that I am only going to use once I just hard code in the values. If I am going to share the model I like to take the extra time to put the parameters in a FreeCAD spreadsheet.
This discussion led me to look up the difference between Parametric and Direct modelling. You need to know more than I do about 3d modelling to understand the explanation.
Your parameters don’t need to be in a particular place. For a finished project it may be nice to have a spreadsheet with the particulars, but it’s fine to have many relative constraints (e.g. these things should be equally far apart) and figure out the naming of your source parameters later. You don’t need to move them to a spreadsheet, either; constraints can be named, and it’s not uncommon to use a master sketch that doesn’t produce solid geometry but only exports parameters (set constraints) and consequences (reference constraints).
The point of a fully constrained sketch is to have it change in a predictable manner when you alter the parameters, so it doesn’t e.g. flip inside out or anything like that. All geometry has properties; fully constrained, it means they all come from somewhere you’ve chosen.
The nicest thing about the sketch constraint system is that it sets up equation solvers internally. That means it can be driven both ways, which in turn means you can leave the decision of what is a “source” constraint until later; e.g. whether you specify the inner or outer dimensions of a box, you can define its shape and wall thickness the same. Doing the calculations in the spreadsheet requires not only formulating the values but solving for the dependent variables.
I have no history of working with 3d modelling. I just watched my son. Many years ago. All my knowledge is dated. I am old and washed up. Boo Hoo.
Yeah, the having a model fully constrained allows you to combine measurements. You can ether set them equal or combine them using calculations to e.g. keep the relative dimensions accurate.
Imagine a perfect cube. You can define all three dimensions separately, but to change its size you need to change all three dimensions. If you forget to change one it is no longer perfect and the model is broken. Combining them into one single definition makes the model perfect, as you can change that dimension and the CAD software will use it to always create the perfect cube.
Constraining and more important naming measurements everything is already a help while constructing. Instead of remembering values you just use names. Sure it takes a little more time to add a human readable description to each measurement, but “TopVaseDiameter” is self explaining and you can use it when ever you need it and all iterations automatically change. When using 125.5 on the other side, you would require to find and change every 125.5, when you want to change the diameter. Especially when there are values like 120.5 which need to be changed, too. They e.g. describe the TopVaseDiameter minus two times the wall thickness.
When done right you just change the diameter and everything is fine, because the other value was defined as “TopVaseDiameter-2*WallThickness”. Without parametric stuff the value would stay unchanged and create an impossible model (negative wall thickness) or a massive wall thickness as the previous diameter is static.
Naming stuff makes easier and is far less work on the long run.
I agree with everything people are saying. Just remember, parametric modeling is really about being able to go back to refine the design. If it’s a 1-off & will never be revisited, I don’t think it buys you anything.
This may be true for a very simple design, but as soon as it gets a more complex part, having named sizes is helping a lot. Especially if you need several iterations to get a part right. It is very rare that you build a part that fits perfect the first time. Usually you notice a design flow and need to change something. Without doing parametric stuff the changes are high, the the change on one side causes an issue on an other side, as some changed measurement did get transferred to another section.
It also trains good behavior when it comes to designing stuff right. 