My old Anni never had 1 service at a stealer - still got the ECU, window regs, alarm, expansion tank and wheels replaced on warranty without question.
Liz's old MKIV had numerous things done, The C30 has had warranty work done, My Monza's are in the process of being replaced and it's had a new armrest and ECU updates... all without question.
It's all a con mate, so long as it's somewhere reputable you're fine. This isn't a hard sell on Votex, any specialist is the same (well, maybe discounting a couple
)
Of course, if it's a heap and has a book full of ebay stamps.. they're gonna ask questions - but a well looked after car doesn't even blip their radar, they're billing everything back to VW.
The lad who came to buy my Anni turned up, looked at the car, looked at the folder full of Votex bills and just gave me the money without even driving it. Didn't haggle - residuals, schmeisduals. I got top money for that too, in the credit crunch! Her Golf sold over the phone.
The only thing you're doing is closing down your market to an enthusiast buyer who appreciates quality work rather than Joe Public who wants a FVWSH, even if that means the work has been crap and the car unloved... so it can take a little longer to find the right buyer, but I don't believe it affects the price.