[Fantomen]

I'd say that ideally you want to focus your offensive mage power in short deceicive wars. If you use your mages liberally in battle, and that gives you a quick victory, you'll get more research time in the end than if you are scared to use them.

Thats why it can be actually good to have research centers close to the fronteirs.

Caelum is a good example of a nation that can almost avoid this balancing act. The enemy gets within reach and them wham! Overwhelming swarm of battlemages from all directions. Next turn they're back to research. I try to mimic that with other nations too in different ways.