20+ years in the ATC game, and can't ever remember denying a clearance, maybe a short wait(1-2 minutes whilst getting or making up a flightplan) and that is pretty rare too.
To the GA guys out there, do not be afraid to call up ATC. We are there to assist, nearly all have an interest in aviation that got us into the job in the first place. Controllers are also GA pilots, gliding enthusiasts, skydivers, airline passengers,ex RAAF, etc etc.
Whilst we're on ASA running Towers in the US competitively, (a rumour doing the rounds),maybe the quote we gave neglected to include a remote allowance of around 20%, and we are actually losing money on the deal.