I have never had a problem clipping in while going up hill... I have trouble clipping out while going uphill. I can't pause pedalling long enough to twist my feet out and I fall over. This tends to make me never stop on a climb.

Oddly enough, it was getting the clipless that really gave me the confidence I needed to make it to the top. With the toe straps, I would yank me feet out and stop with the least amount of pain. Then with the clipless I couldn't stop and I learned that I really could make it to the top.