I've never nicked a dog with my #40 blade on the feet, only testicles since the skin is so.. weird. (I think the shorter cut on dog balls looks better.. so I use a short blade)
Unlike my other blades, when I use a 40 I don't press against the skin, but rather hold it against the skin if that makes sense? I know a lot of groomers use a 15 for feet and faces, but when you hold a 15 and a 10 tooth to tooth with each other, their teeth are just as wide apart with the 15 having the blade closer to the edge. This is MUCH more dangerous. Wide teeth with the teeth close spell big problems, the closer the teeth toegher the better. I like 40s because the teeth are so close I can't even get a fingernail between them.