I would think it is okay. I know many sources don't recommend long hot baths for pregnant women, but others say it is fine and I would think the same would go for ttc. I think the ones that say no hot baths during pregnancy are referring to the effects of hot tubs that can raise a person's body temp. when used for long periods of time. In a normal hot bath, however, the water gradually cools naturally as heat is lost to the air and the tub. The water wouldn't likely have the continuous heat necessary to elevate the body's temp (unless you continuously run new hot water into the tub after it has been filled). I believe Sheila Kitzinger discusses this in her Complete Book of Pregnancy and Childbirth. Again, she is talking more about pregnancy than ttc, but I think one would be more strict during pregnancy than ttc, so I think it's relevant.