I've never had a problem with a timber rolling out, I could see it happening if you don't have enough wraps or if the trunk is uneven enough the wraps don't press against it. Both my PW's have VERY long slings and are cow hitched 99.9% of the time. My blocks, usually cow hitched just to eat up their extra length for the most part. But I have no concerns using a timber hitch, just make sure it's dressed and set correctly. I bought a whoopie sling a few years back, but those are pretty much a joke, way faster to tie a single eye sling than set a whoopie, IMO.