Jun and Chuck,
In travelling, my experience has been that most places have a mat fee, but rarely have I had anyone accept it. Our own dojo has a fee with about the same frequency of collection.
Regarding etiquette, I've always messaged on ahead a couple months before I plan to attend another dojo. Part of this is selfish - I've found that schedules obtained in printed materials, and now even the net, tend to become out of date quickly, so I want to be sure that there is actually a class, as time is usually limited when I travel. I used to mail ahead, now I email, and really appreciate it when site like Aikiweb offer links for email and/or web sites - it really facilitates the process.
I usually ask about times, mat fees, and what is considered acceptable uniform, so as not to come to the dojo "underdressed" (if I can leave the hakama home it makes packing easier).
I've always found warm welcomes and fun times. In Ireland we were asked to the pub after class, for instance.
Until recently (in my ignorance) I was not aware that not only is it a good idea to message ahead, it's also good protocol. I observed this when an aikidoka from another town dropped in to watch practice unannounced, with a bit of swagger, to our home dojo which is small, friendly, independent, but traditional.
After he viewed class and left, our sensei, without directly criticizing, talked a bit about the politeness of forwarding a message ahead before you visit. Perhaps, this is a vestige of the old-time "letter of introduction." I have no problem with it, and think it's a good idea. Actually, when I visited one town, the fact that it was known I would visit, enabled me to meet the head instructor (who wasn't teaching that night) who is well known as both teacher and author. I was humbled as he walked in, stopped by the side of the mat, and asked if I was the one who had written earlier, and introduced himself. Pretty neat I thought!