I've never heard of tipping farriers!!!
I don't know if anyone else does but we don't . . . .it cost enough as it is without a tip on top. Also I don't think a lot of farriers would accept a tip after already being paid for the job.
At a big show barn I worked at, customers would tip the farrier's aid for holding their horse occasionally, but never the farrier for the work he was there to do...the way I see it, they build their tip into their rates :)
I only tip if he makes an extra visit to the barn for me specially, otherwise I hate my farrier, I think he's a jerk but he does a superb job so I deal with his cocky attitude to keep my horse sound since the only other fellow over here who does a good job charges fifty bucks for a trim, which I think is ridiculous.