I asked that question at indoc and was satisfied with the answer of balancing both schedules. I asked that about someone going to UPT after being hired, and the impression was for them to get some use out of the person before they took a year and a half leave of absence. As far as taking months away for upgrade, I would try to delay the class until you have at least a few months of work time in. They do spend $$$ for the type and wouldn't want to have to send you back for recurrent without any real return for their investment as a new hire. I also hear of an A-10 guy who just returned after a long absence.
Bottom line is that you will NOT be paid during those months. (someone correct me if I'm wrong on that)
Hey Faipsrule, where are you a reservist?