Tips shouldn't be expected or asked by the provider, imo.
It's up to the customer to decide if the service was "good enough" to justify giving an extra reward to the crafter.
If I get a crafter, who does his work, and he proves to be a nice person, I pay his crafting fees, plus a tip.
If I get a greedy provider asking for a tip (besides his fees), I don't pay the tip.
Sometimes, I get one of those crafter who work for free (for skill points). In those situations, I'll try to tip him, for his effort.
Tips are optional.