Are fluids necessary for a dog under anesthesia?

Next week my dog is going to have his teeth cleaned. The current vet (who is going to do the procedure) says that they will draw blood in the morning and check 6 parameters for any problems and then do his teeth in the afternoon. They say if the blood work is normal, and since the dog will only be under for 10 or 15 minutes, that fluids aren't necessary. They said that they may decide that day to put him on fluids, and it will cost me $25 if they decide that.

Money is tight, but I dont want my dog to be uncomfortable and/or have complications. Should I just request they put him on fluids as a precaution?

My guy is a beagle mix, 5 or 6 years old (a rescue) and seems to be in good health, but does have pretty heavy plaque on the teeth and some of his teeth are ground down, so I am also wondering if the "10 or 15 minute" estimate is a little too optimistic - are fluids more important if he is knocked out for longer?

Any vets or vet techs out there know the true importance of fluids and what are the pros and cons of using them?

Also, my dog has a history of seizures (one every few months). I am not sure if it is a concern with the anesthesia - I heard dogs can have some problems when waking up. I plan to remind the vet when I drop him off in the morning, but I am wondering if I should call ahead to remind them, or if this will just be annoying.

Thanks for the help!