I played every game of the first two seasons since I wanted every skill point I could get. Now, in my third and up to an 84 ovr, I only play key games and usually play 12 min quarters and pursue milestones. Whereas before I was pulling like 1K per game, now with key game and milestone etc I can pull 10K in a good game. In simulation the cpu has been putting up realistic stats for MP (even the turnovers!) so my playoff/all star campaigns aren't damaged in simulation. Also nice about playing key games only is you get mad drills to do, which gives you more points, and you develop your player faster.
It's a matter of your personal preference and strategy.
Make sure you check out the "official My Player thread" here. IMO it should just be a sub forum, it's like 400 pages! Lots of help over there.