To be honest there is no hard and set rule, what ever works best for you, In time you will develop your own preferences.
Be patient, dont expect to produce outstanding models immediatly, it takes time. The best advice I can give, which was given to me, spend as much time looking at what you are modelling as actually modelling. It does seem a really silly bit of advice but if you apply it you will see a huge improvement in your work.
The greatest thing about producing models is that we work with polys, if you mess up it usually can be very easy to it right.
I give ip with thos baner fing as it never seems to work for me. I woll now stop wasting mu time.