Nose hairs from completing heroic

For AAT: If you've already gotten 2 AATs in this week (Sunday thru Sunday), and expect a 3rd with your home guild, then don't jump to Merc. If you plan no more than 2 with your home guild this week, then by all means go for it. Also, the rewards will be there in either guild, even if you leave before payout (you can always come back and get them). Further, you can jump guilds once every 24 hours. The only thing you'll be affected from is donations for 24 hours after joining (each jump).

For Rancor Heroic: Everything same applies, but you can do up to 4 in a week.