Nothing immoral about more than 1 seat deposit, IMHO. But here's some info that might be of interest. B4 June 15th, law school admissions people can find out real time how many of their admittees have seat deposits at other schools. LSAC will tell them (eg): of the 275 who have given you deposits, 120 have deposits elsewhere, too. On June 15 or so, LSAC releases names and schools. So: after 6/15, each admissions director knows the names of every 'seat depositor' who has deposits elsewhere, and where those deposits are. At that point, you'll likely start to get calls asking for what you're thinking, how you are leaning etc.
One poster above notes that some people hold multiple places til the end, and that is true. But understand that many 0Ls need to move to a new city, find a place to live, figure out transportation, etc. If you don't release your slot til the last minute, dorms might be full, and the wait list person who gets a call may have to live off campus. Would you like to be figuring out which neighborhoods are safe to live in, have good bus routes to the school, etc in a city you don't know 3 weeks before classes start? I can't give you a magic day when holding multiple deposits becomes mean/immoral, but I think you can see that the golden rule comes into play here. Suppose there was a school that is a little better than the best one you are in now. Surely there's a point where, if you got the call from that school, you'd say: I'd prefer to go there, but it's not worth the hassle. Sure wish I'd've heard a week ago. Plenty of people are in that position, and you (or the child of the people you spoke to) should show some consideration.