I don't think it's absolutely neccessary. Sure, you will probably be much better if you do shoot year round. My schedule is so crazy the last few years...I never know when or how much I'm gonna get to shoot. Sometimes it's alot more than other times. I can honestly say....that I know I could go out and kill a deer within my normal range (under 25) without any practice at all. It ain't rocket science. Once your bow is tuned correctly, and you have been at it a while....it's almost second nature.
Would I ever do that? No way.
However, I have had years where I've been so busy I haven't been able to practice much, and I was still very accurate, and grouping well right out of the gate.

3 years ago, I spent every weekend for about 8 months, and many weeknights building my house. I moved in on Sept 25...1 week before archery season opened up. I had an honest 5 days of practice (probably an hour each day) before the season opened. First day, 22yd double lung on a doe. I couldn't have made a better shot on her if I had practiced 500 shots a week the whole year prior. So yes...it can be done, but I personally prefer to shoot as much as I can. Like others have said, I like to shoot, plus I feel much more confident knowing I've been shooting regularly most of the year. Most years I shoot from about April to October a few times a week, then it tapers down during the season to about 1 time a week, then after the season when time and weather permit, I will shoot. Time is probably the biggest factor for me anymore.