Best practice: Modelling a part in relation to the assembly or not?

(I'm new to CAD)

I started what I thought would be a straightforward modelling project and I quickly found that I'm stumbling on how to "think" of parts spatially. I have a machine that has a left and right matched die set, the dies come together to seal a bag. I need to model these dies so I can make some modifications and make a backup set, it's also a good OnShape exercise for me.

The problem I'm having now is how to position these parts (not assemblies) in OnShape.
Is it more common to model:

1. Relative to the operator/mechanic of the machine (i.e. how you commonly look at the parts and visualize them). In this case I would model my right die as it is installed in the machine, and my left die would be modeled in mirror.
2. Or is it more common to think of parts as free standing things, without context or orientation?

I started with number 2 as a result of physically measuring the die on the bench and drawing/dimensioning it on paper. I gave no thought to the orientation of the part to the machine as I was "in the space" of my 2D paper drawing. When I began thinking about creating the other die in OnShape that is when my eyes started to cross, I walked out to the machine 10 times to look at it and was trying to figure out how I should position and orient the part in part studio.

Anyway, this seemed like a newbie thing and I thought I'd ask here. I hope I've explained the problem clearly enough and that some of you experienced modelers know what I'm talking about and have been through this.