It's definitely a good idea to blow the water out using a compressor. Since naturally there will be sections of the system where water will sit, and the pipes are not very deep (typically 1 or 2'), they will likely freeze, and if that happens the pipes will crack, and you'll have a non-working system (and lots of digging to do) in the spring.
Check this question for advice on how to connect a compressor to the sprinkler system. It's best to blow out each zone one by one, so that you don't end up with water stuck in one line still while all the air escapes out another already-empty zone. Start with the zone with the highest elevation, if possible, and go down from there. You may need to cycle back through one more time.
Once you have the fittings ready to connect your compressor to the system, it's a pretty quick job, should only takes a few minutes really. Once there is air coming out of the zone, you're good and can move on to the next.
50 PSI is about the max you'd need on the sprinkler system side. Be careful going higher - 100 PSI is probably higher than the pipes/fittings used for sprinklers (or the sprinklers themselves) can handle.
