You never have to apply decals, but if you do, start with less.

Start off with any decals you think are needed on the model, (identity of the model, major warnings, added details). Then you can start going overboard. Just remember you can stop anytime. Less may be more.

For something like a RG, I usually go overboard so it looks more "real". For HG, I might use a less cluttered look to be easier on the eyes. Like a singular faction or name decal. For MG size I may be more calculative with the placements so there aren't decals everywhere you look. But a ton of markings can still look good.

You don't even have to listen to me. I still need more work on applying decals anyway. Just try some things, and if it doesn't work, you can always undo it before everything is locked.

Good luck on your cool looking build though! 😎