x1achilles

I fly 4 times per year with my dog, a 15lb (6.5kg) Boston Terrier. I have never had a problem with check in or weight. In the 20 or so times I've checked in with United, Delta or KLM they have never looked at the dog so much as weighed him. My dog is really well behaved and never makes a sound. If he can see me, he's happy anywhere. He also has no problem with not relieving himself for 10 hours if nec. The zipper on the carrier has been completely broken for two years and no one has ever commented on it.
On a flight from Puerto Vallarta to LAX in April 09, the passenger next to me begged me to show him to her. He snoozed on her lap nearly the entire flight, the FA even commented on how cute he was (though I wouldn't count on a typical FA allowing this.)
You are not allowed to walk the dogs through most airports. Once in the gate area, you probably can let the dogs out for a bit if they are well behaved. In short, the check in process is the easiest part of the whole trip. Good luck and don't worry so much.