Bottled beer is the way to go if you have suspicions about the place not being very good or if it is not a beer bar. Draft beer, when the keg is open for too long, is not good. Shady places may add some stuff you don't want in your beer. In all other cases draft it is.
That's an excellent point actually, another issue can be if the beer lines are not kept clean, you can tend to smell it in your beer. But yes I'd always start with a tap beer and if it's off in any way switch to bottled (or more likely go to another pub ha ha)