There are three types of trash skipping.
1. The out of the way trash. I don't go out of my way to kill trash if I'll just have to back track to where I am now to get to the boss.
2. Trash thats on the way to the boss but you can hug the wall to avoid it. This trash is not a waste of time from an XP/time ratio and if its not difficult or annoying for some special reason I see no point in skipping it. I think people only skip it because thats what they've been taught in heroics and they're not capable of thinking for themselves.
3. The "i know a trick to skip this pack" pulls. These are the spots like in slavepens where you can jump down from the bridge and land on a little ledge, then climb along the ledge til you get to a spot where you can jump down to another ledge, then jump down right into a pack so you all have to do it at once. Someone always falls so you have to wait 2 minutes for them to run all the way back around and try again and all it does is skip 8 easy mobs. If it works perfect you save 30 seconds. If someone falls(which ALWAYS happens) you waste 2 minutes. These "time savers" would be retarded even if you didn't get XP for the kills.
If you have a meticulously planned questing route and you're not leveling with the RAF triple XP bonus then questing might be slightly faster but if you aren't following a step by step, quest here, skip this quest, do that quest type of guide or you have constant triple XP from RAF then running dungeons is the way to go and killing trash is not a waste. I'm close to having 9 level 80s now(+1 bank) and 6 of them went from 1-60 under refer a friend. We tried questing and we tried dungeon crawling and the dungeon crawling was about twice as fast for us.