I try to avoid using the mainsheet while the spinnaker is flying. The sail acts like a backstay for the mast providing a great deal of support. If it is windy enough, you could fold your mast without mainsheet tension. I ease the traveler instead after coming through a jibe...just a little to help things breathe and then quickly bring it back as the boat re-accelerates.
I agree with the backstay comment. I do exactly the same however lately I've been experimenting letting a little traveller out, especially in big winds. We seem to be able to run a little hotter but the boat is far more settled. I'm not 100% sure if VMG is better overall though just yet.
Tiger Mike
