It's always good to rotate sites for one reason in particular...SCAR TISSUE! If you build up too much scar tissue in one specific site the tissue will get hard and very tough to push a needle through. It may even effect how well that site can hold oil. I've been using 23g pins for my delts this time around because it's all I have, but I have noticed a steady buildup of scar tissue and now I have to take a couple weeks off from those sites to let them heal. The more sites you have to work with, the less chance of scar tissue buildup. For those of you who have ever stuck a needle into scar tissue, you know it doesn't feel good. It's like hitting a brick wall and pushing oil through it can be painful and near impossible.
The site that most people underutilize is the ventroglute. Do a quick google search on ventroglute and you'll see it's a virtually pain-free site and is much easier to reach than the UOQ of the glute. Better yet, check out spotinjections.com