I myself do a bit of both. I paint plastic models with enamel paint. Theres positive and negative for painting as you go and painting after construction.
When i paint bit by bit and than do the glueing i find the pieces dont stick together as good as what they do if they are unpainted. But than again i use plastic glue which is meant for plastic and not enamel...
But than if you try to paint after everything is glued than your inevitaly going to run into cases where things are just rediculously hard to paint. An example is a spade on the tank im making. If i tried to paint afterwards it would be rediculously hard and time consuming, so i paint it beforehand.
I believe that the best thing you can do is try to achieve a happy balance. Just try painting things at different stages and see how it works out for you. Eventually you will find your footing
