All of the websites I have seen warn against using WD-40 in sewing machines. I can understand it not doing any harm if you are able to remove it all. Most people won't have an air compressor handy, though.
Sewing machine oil is the safest thing to use.
I don't use WD-40 for a completely different reason -- I can't stand the smell of it. My husband used it once on our outdoor chairs to stop a squeak. The odor never dissipated; finally had to ask dh to remove as much of it as he could and never use it again on anything I would get near! To me, the stuff smells like poison. I just figure anything that smells that bad to me is probably toxic (at least to me!). Doesn't seem to bother other people -- dh wasn't bothered by it at all -- but the stuff makes me shudder even as I write this.