I agree with the advice about pushing/pulling two stops or so, but not more unless you're prepared for aggressive results.
For me, let's say if I only have a roll of iso400 film (which is never, hehe):
- In bright sun, I use iso 250
- In normal or lower contrast I use iso400
- "Golden hour" images that need to be handheld, iso800
- Interior photography with challenging light that need to be handheld, iso1600
And then I use the massive devchart for hints on how other photographers adjust developing with the chemicals I use.
The best approach I think is to first shoot at the manufacturer's recommended settings and develop the film to their specs, and see if you like it. Then experiment and have fun with it.
Another "best approach" of course is to always have more than one kind of film if your refrigerator is big enough :-) Using films like Neopan 1600 indoors and Plus-X 125 in the Sun is a good approach. I usually like the results better if I use fast and slow films as they were intended, as opposed to forcing one film to do too much. The problem with that approach is that the films have a different grain structure and that bothers some folks.
Welcome to Filmwasters!
Cheers,
Jeff