The person that has their map all done, and is requesting a couple models to polish it, is about 10 times more likely to get their request done before a requester who hasn't even started his/her map yet.
Another couple things about this is, don't use the excuse that "If you make the model, I'll probably get the map done!" This is one the lamest excuses and will probably make you getting any request taken very unlikely.
The map comes first. There are plenty of Warcraft III units to use as temporary units to use until you are done with your map. Then come back and make a request.