We do a free bug-prep spray on all cars that come through. If they are truly "covered" (and not just what people think means "covered") we will take our prepgun to it as well. If we can tell that a customer is going to be picky about every little bug (as in, they ask for extra spray, etc) I have my loaders trained to hit the front with the prep gun.
Now as for the complainers.
On my higher end washes ($10-$15) I will take a look and gauge their mood. If they aren't being jerks I explain to them the difference between an Express Tunnel Wash like ours and a full-service or detail wash and what happens with bugs and why our wash didn't get them. These people I will normally offer to send through again, about 50% take me up on it, the other 50% understand once I explain to them it probably wont take any more off.
If they are being jerks I still explain to them the differences and why the bugs didn't come off, but not rewash. Especially because most of the "jerks" will complain about only one or two bugs, and I'm not rewashing for that.
Customers really don't want to hear about "clear coat" or "express wash" though, they just want what they think is a fair wash. Best we can do is explain why they still have bugs, best way to remedy it, and hope they understand.
Now for bird poop, that's a different story. If there is a lot of it I will gladly rewash. One or two little spots on the hood/
door/windows, that gets a quick spray of windex and a dry paper towel. I wipe it for them, and apologize for not getting it all off while also explaining why two week old bird poop didn't come off very easy, and 99% of the time they're gone in minutes and it costs us next to nothing.