No, it’s not the standard. However, it is awfully handy to be familiar with both PC and Mac. I mean, you’re bound to run across both.
As far as taking steps towards a career goes, I’d learn the programs in and out. Learn everything you can. The more tools you wield, the more valuable you are.
Practically the Total Training series for After Effects is just amazing. Very thorough. It’s nice to have an instructor go through the entire app with you.
If they’re a bit pricy for you, buy an older set off of ebay. 90% of the info will still apply (just don’t go too far back).
There are also a decent number of fantastic tutorials posted up here.
Aside from that, the manuals are a pretty obvious source of knowledge. The best way to learn, by far, is too actually use the program. Always be working on something, even if it