Saltwater tanks can be done sump-less, but as techen said if you can fit one and the floor will support the extra weight, it will make your tank easier to manage and more stable in the long run. The sump gives you both more stability (more volume = more stable chemistry) and an easier place to put stuff like a skimmer or a refugium, which otherwise might end up as HOB things that are harder to reach for maintenance. If you like a lot of fish, sumps can also greatly increase the bioload of a tank beyond what it would be un-sumped, although a good skimmer can also do that on an un-sumped tank. However, if your tank is not drilled, adding a sump is a more fiddly business since you would have to do an overflow box with a siphon tube. I just went through that process with my un-drilled 55gal (which is a few years old and was un-sumped until this month) and it is a non-trivial thing that requires a reasonable amount of planning.
Sumping is probably only worth considering if you can fit at least a 15 or 20gal to have room for equipment in it. If you only have space for something like a 5gal, then you might be better off using a canister like techen mentioned for chemical media or extra rock rubble - or even skipping that altogether and just getting a really good hang-on skimmer.