As long as the water is clear, I use it unless I'm changing more that 20%. I'm more careful with larger changes.
I use one of these to mix it
http://smile.amazon.com/gp/product/...powerhead&qid=1452303502&ref_=sr_1_11&sr=8-11
It's one of my DT powerheads. The way it attaches to its clip you can just pull it off and let it hang down into the bucket to mix. You can get them for like $6 on ebay I think.
Those food grade labels are just a thing you can buy. The HD buckets not having them doesn't mean there's anything diff about the plastic, just that they didn't pay for the stamp cause they aren't making buckets for food.
It's a common idea that there is something better about "food grade" plastic, or certain recycling codes, but I'm pretty sure its a myth. There certainly are dangerous things in plastic, but I don't think food grade, or the numbers, tell us much about that at all. After all, PVC passes neither test but we use it all the time, and BPA's are common in "food safe" plastic even though they are terrible for you.