I've read that over steaming can cause the inside of the bend to buckle but I've never had a problem with oversteaming. That said I never steam longer than an hour because that's when my steamer runs out of water. I have steamed 3/8" pieces for 40 min which would be about twice the 45 min for 3/4" recommendation. No problems. Most stuff you read is for tight bends for furniture. Most of the bending we do, other than sharp recurves, are fairly mild bends and I don't believe they are a problem.