I tend to buy anything I think I may need but skip stuff I KNOW I won't. For example, if I'm not playing an archer and know I won't convert a character to an archer I obviously won't buy archery skills. As you say, skills are cheap and money is plentiful.
The battlefield merchants sell pretty much anything you need - at a high price - so if you miss buying something you can catch up even if you need to wait for the next act for the merchants to upgrade their selection of skills.
I don't recall a skill I needed that a BF merchant didn't sell.
