wow, i didn't think anyone still used regular straps these days i use my tree straps by doing this, i run one loop through the other and girth hitch the tree. i started doing it because i was also experiencing them sliding down the tree when done the traditional way. this way is far superior as far as i'm concerned and it's easier to tie a knot to one support loop than to tie the lashing or a knot to 2 support loops at once.
i think as far as wear is concerned, the spot where the girth hitch is, (the webbing on webbing part) is ok. i don't do this in a climbing situation, but i've never noticed any wear from it in this application. there's nothing really moving in that spot so it doesn't seem to wear to any noticeable amount.
the line to tree strap spot is a different story though. there will be wear on the loop(s) if you tie directly to the webbing weather tying to a single loop like in the girth hitch method above or if tying to both loops like the hh method describes. check the loops regularly and retire when necessary
the best way to minimize the wear is to tie a knot that only lets a minimal amount of line move through the webbing as it tightens down under load. i could see the lashing method letting lots of line slip through the loops as it tightens. i suspect that may be the reason behind many of the cut tree strap incidents but that's just a guess. also, make it a point to take the weight of the hammock off the line with one hand as your pulling line through the loops when setting it up. it's not alot of weight on there, but you're pulling several feet of line over the webbing loop just to get it where you want it.
an easy way to eliminate the wear entirely as others have pointed out is to put hardware between the line and the webbing. you can easily girth hitch a single ring to one end of your strap if you don't want to sew it in. i started adding a single welded tri ring to one end of each tree strap i sell once i noticed signifigant wear was happening with the traditional ringless straps. a welded steel ring can be as light as 8g so you get alot of long term durability and peace of mind for little added weight.