If you're not using any SSI inclusions it's best if you leave it as .html
.shtml is only if you are needing to use Server Side Includes on your page...
It --will-- work either way but will allow you a millisecond faster download speed and save the server from extra parsing work if there is no need for the server to be parsing together the pages...
e.g. only use .shtml if you are using SSI otherwise .html is recommened
Deb