I've tried doing this stuff both ways to be honest mate, in the render set-up and as a post-pro thing. Personally I find doing it post- in Photoshop is, for me, much faster, and allows a lot more flexibility to try out different options. Best of all, it allows for those 'happy accidents' when you layer stuff up and suddenly find an effect that just 'works'. That said, this is also partly a reflection of my lack of real in-depth rendering expertise, and I'm always in awe of those people like Solo who seem to get amazing results straight out of the render engine, with minimal post-work.