I have a 20 gallon reeftank, or soon to be reef tank (once it matures some). from the research I've done it seems a 5 gallon saltwater tank can be done successfully if done properly. usually they use a lot of live rock and live sand and a power head and an extremly light bio-load. as a general rule i don't think Nano tanks are good beginner tanks.
however they are easier to maintain than a bigger system. so i guess it all depends on wether or not the begginer takes the time to understand whats going on in their tank or not. a 10 gallon tank with a bunch of live rock, 2 very small hardy fish and a cleaner crew would be pretty self sufficent after cycling.