I fullly agree with Jen since they are catering to you for their services on the day. I prepared thank you cards to give our vendors. The only that received a gift was the wedding coordinator at the venue taking care of everything for breakfast and reception.
Oh this is a great question I was wondering the same. The girls are right (based on what I have read) - if they own the company a gift would be nice, if they are an employee - cash tip is best received!