After getting a direct routing a small show of appreciation would help, and also ensure a repeat. Swissair kept Maastricht UAC in chocolate for many years, and a seat on a Danair flight was also guaranteed
That aside, most controlers will always do their best to give a direct routing, if only on the basis that it will get you out of his/her sector ASAP and let someone else worry about you. If your request is denied, that is the end of the matter; whining about it will probably only get you
special treatment