Guess I'm lucky. Been dealing with the same repair service since moving here in 1970. Have bought several machines from them and they do all my service work, even machines I bought on Ebay at a cheaper price and a vintage Singer I bought at a garage sale. . They have ALWAYS call befrore doing anything I haven't specifically requested. They always provide a itemized list of every thing they've done and the cost - of course the labor is always the most expensive thing on the list:>).
Shop around for a service company with a good reputation:
How long have they been in business;
Many have letters of thanks, recommendations, certificates of awards, etc on display;
Contact quilters in your area for references (fabric stores have bulletin boards with business cards of quilters and guilds, they're glad to help);
Don't be afraid to ask questions;
Can't made a quilt in the store, but give it a "once ove" before you leave the shop - you would have certainly notices 6 new holes;
CHECK THIS SITE FOR MEMBERS IN YOUR AREA AND FIND OUT WHO SERVICES THEIR MACHINES!!!