Unless you do something really stupid, you are very unlikely to ruin a model just painting it
After all, if you think you've botched the paint job, you can always try again ? either just paint over it, or strip the old paint off first.
This, like many other things in modelling, is a matter of both experience and personal preference. The more models you build, the more you'll develop your own ways of doing things. The best advice is to take everything other people tell you as advice
, not as gospel.