I have done too many 'after the fact' kitchen remodels. Sure, the poor customer is just trying to do things right, but they usually get some bad info along the way.
The fact is, that during alot of kitchen remodels (where the sink is staying in the same spot), the plumber is the last one on the job. Usually, the only reason we even get there is that the cabinet guy can't make the new setup work. Few and far between are the jobs where the plumber is seen to be integral to the remodel, unless it's rather significant.
I remember one job in particular. An individual had new cabinets and countertops installed. Looked beautiful, nice granite countertops with undermount sinks. Probably dropped a good $30K in the kitchen. Only problem was that the drain was about 6" too high.
So, the cabinet-fool was nice enough to trim-out all the plumbing for the sink: the dishmachine, the r.o. system, faucet and disposal. He left the customer saying "I don't have the part for hooking up the drain, so call a plumber. It won't take long."
The guy should have been a comedian.
Well, 7 hours and $1200+ later the customer had a sink they could use. It was the day before Thanksgiving and it needed to be done. If they had called before 2pm, it would have been cheaper.
Every problem imaginable: waste piping was lead, rusted out flat galvanized vent, hole for sink drain (in plate) also used as an access for electical wiring installed with the remodel.
What would have been a 2 hour/~$350 job without the cabinets installed became that.
So, yeah, having a plumber come out, even to say 'Yep, looks like it'll work' can save you quite a bit in such circumstances.