Believe the performance graphs, and calculate them every trip, especially if it's hot, wet or low pressure or you have full tanks. I know a very experienced pilot who did not on grass in a Warrior, 4-up and nearly did not make it. Could have been embarrassing for him, considering his position (not with my club, I hasten to add).
With this caveat, yes, I would think it is useable (have flown them from grass, though not often).