If you are flying you might need to do some tipping before you get to WDW--the shuttle driver from the parking lot to the airport. The skycap if you use his services. Both generally get $1 per bag, more if the bags are large or heavy.
if you are taking a shuttle or car service from the airport to WDW, then you'll tip the shuttle driver (MEARS) $1 per bag. General concensus is that car service drivers get $10 each way, a little more if you have a lot of luggage or make a grocery stop.
You'll also need to tip if you use valet service to park your car (I tip $2 on drop off and pickup), and tip $1 per bag if you have a bellman help with bags if you use those services.
If you are staying at a value resort, I don't think the luggage services people there are allowed to accept tips, although I could be wrong--if I am I'm sure someone will correct me...
Other things you might need to tip for are the obvious, bartenders and servers. Standard is 15-20% for good service, generally service at WDW is great and warrants that 20% tip
If you plan on spa services a 15-20% tip is standard.
You don't tip front desk people, hosts/hostesses/seaters/greeters whatever you want to call them at WDW, they can actually be terminated for accepting them.
Housekeeping was already touched upon. Can't think of any others...
I get $50-100 in singles and another $50 in fives before I leave for a WDW trip. Sometimes we go through them all, sometimes we have quite a bit left. It seems to vary from trip to trip, a lot depends on where we stay and how long our trip is.
Have a great vacation!
Anne