I've seen UTF-8 with a BOM while consuming data when integrating with strongly Windows-centric environments. Relatively uncommon, but does happen. And it is very annoying!
BOM is only a problem with strict syntaxes, which robots.txt is not an example of. If the "consumer" simply ignores invalid or meaningless lines, you can avoid issues from invisible characters by not having anything meaningful on the first line of your file.
Which is why all my robots.txt files have a comment on the first line.