If you're trying to model a vehicle accurately, one of the most basic features of any vehicle are it's markings. If, however, you don't care, and are just modelling for youself, then leave them off.
I've left decalling off of many of my models, because I just wanted a model of a particular vehicle, and didn't particularly care about what division/time frame they were from. At the least you should at least provide country markings.
You'll also find that the markings add a LOT of life to your model, as they provide relief from the camo scheme, and something different to look at!