My experience, is it gives you a "Nag" message every day, but you don't have to grab a new license file. You can just say use existing license anyway.
Yes, you only need the subscription when you update your Quartus version. Quartus 12.0 was just released, which provides better support for Stratix/Aria/Cyclone V families. But if you are not using these families yet, you can probably hold off for awhile.
Cyclone V looks like it's a very good part, with great power/performance metrics, but Quartus 12.1 will support more members of the Cyclone V family, so if you can live without it for awhile that's probably the version to get the upgrade on.
If you are playing with Cyclone V in Quartus 11.1 SP2, please note, that the timing models in 11.1 SP2 are very conservative, and 12.0 will give you much better results.
Pete