I'll add two things:
1. If you're going to do it, I'd recommend getting one rated 'for tablets', IE they have the ability to deliver 2.1A to individual ports AND tell the tablet that it's capable of high power.
2. Realize that many of these come with significant standby drain, IE they're still drawing a watt or so when nothing is plugged into them.
For example, going by the picture it looks like you're going with a
Leviton Decora 15 Amp Combination.
3.6A charging capacity indicates that you're probably best off only putting 1 tablet on it. It's 1A per standard port, 2.1 per tablet. So I could charge my phone and tablet off it at the same time.
I can't find the information online, but last time I went looking I found that one product used 5W as standby, as a comparison TVs are only allowed .5W today.