I definitely think it is a barrier line for some women where the licking suddenly creates too much intimacy and they feel it becomes too attached or sexual when the licking comes into play. I've had one really hot fitness model specifically agree with all the content i requested to do with her in a shoot ("except for the licking part" as she put it). I also think the idea of slobber will be a barrier from the standpoint where they become grossed out over the notion, but I've repeatedly found it at least equally (but often more) a gross out notion (to 1st time women) when I start inhaling at a newbie's toes though.
I'm a fan of sniffing and licking but each has it's disadvantages. With the licking, there is that smell that gets tainted and taken away when you lick up that particular surface area. And with sniffing, the downside is that you have to exhale at some point and the pleasure is interrupted so to speak compared to an act of pleasure/sense like tasting, touch, or sight (which are choose-ably continuous, whereas inhaling, aka sniffing, is forcibly stopped at lung capacity after the air has to be vacated).
My favorite motto with feet is summed up by a fun little acronym:
L.S.S.F.
It stands for Lick, Sniff, Suck, and Fuck. The session indulges all 5 human senses when one engages in
LSSF 