statjunk and bigcountry - Now statjunk, here is a case in point on selecting powders. BigC uses IMR4350 in his 7mm-08 obviously with good results or he wouldn't use it. My 7mm-08 hates it but likes IMR4064.
As for when to use magnum primers, consult your reloading manual. Some powder/loads require the use of mag. primers. I've also found that, once you have a load worked up that is producing acceptable accuracy with regular primers, try loading up a few with magnum primers and see if it improves the accuracy. Sometimes it does, sometimes not. The mag. primers are hotter giving more uniform ignition particularly with slow burning powders.