The beer often does touch the lid though (hence the wisdom of blow-off tubes), and beer is pretty acidic so it leeches chemicals quite easily. You can really only tell if it's food grade if you have some way of knowing precisely the type of plastic being used. However, many homebrew stores sells lids (with the proper hole and grommet) separately, so you'd probably be best going that route.
Also 5gallon buckets are too small for typically 5gallon batches. I really wouldn't put anything more than 3.5 gallons in them, personally, 4gal if you really want to push it.
