That's a big question!
Once you get a job welding, you'll come across a huge spectrum of joints and situations; usually you'll want to weld up the joints in the easiest way possible. It's a hard one to answer in general as they vary from job to job. What works on some projects, won't work on other projects.
The best way to make it challenging for yourself is to work on getting your measurements deadly (bang on!), your welds looking mint and the final product looking pro. :)