SIL fonts are the open source licence ones and I think you can use them for whatever you like with some minor restrictions. No problem converting to Woff etc though.
The ones you really want to avoid are fonts limited to personal use or indeed where you only have a licence for desktop usage. Just because you find a font on a website and it says free, do not assume that means you can use it where you like.
Ideally look for web fonts with extended usage rights. If you are only building a couple sites for yourself you can probably make do with a small number, but if this is for work with lots of clients and job types then you’ll really need to build a large inventory. I find it quite sad when I look at local designers and see they are all using the same bland Google fonts for every project, regardless of the subject matter.
I’ve just checked and at the moment I have 118 typefaces with commercial rights and many of those have a dozen or more variations in weight and style, so it really adds up and no doubt I’ll add more. This is why I really want to see a font manger overhaul in Blocs.
Here is an article that may help explain some things.