I can't say we have ever stayed in a truly superior all-inclusive, but it does seem that tips are appreciated and result in better service at the mid-range places. Most all-inclusive resorts where we have stayed claim that tips are included, but I've found that even small tips (a dollar or two for small services or drinks, maybe $5-$10 for a meal) go a long way towards getting more attentive service or special requests later in the stay. I generally use dollars or pesos, whatever I have.

Since I am incredibly picky about my mixed drinks, I always tip the bartender or waiter for each drink; that seems to help them remember my special requests. (Top shelf liquor instead of well; I don't want sweet and sour mix, I want fresh lime juice and no simple syrup in my margarita. Extra lime juice and just a tiny bit of simple syrup in my mojitos... etc.) Getting the right drink every time? priceless!

We stayed at one all-inclusive resort that had a capacity-control policy (something like you could eat at the nicest restaurant once during your stay), but we left a nice tip ($20 or so) on our visit the first night, and had no problems eating there several times during our stay. We would just stop by and ask the hostess if there were any table available, and magically there would be one... YMMV, or course.

I always tip the maids at least a couple of dollars per day, more if the room is a large suite.