... are almost always a bad idea.
Naturally people would like to know what sort of expense they're about to incur, so they want a dollar figure without having to drive all over town. When I am forced into it, I almost always aim high, just in case. If it comes out less, people are happy. But going high is a good way to lose work, too. It's a tough balance, and usually I will say that I'm happy to give an estimate if I can see the instrument, but not over the phone.
Case in point -- I do repair work for some of the local music stores. One day I received a call from one, asking what the charge was to install fine-tuners on a violin. I mentioned two options (i) installing 4 fine tuners on the old tailpiece, and (ii) installing a new tailpiece with built in tuners. The second option is almost always better -- the assembly is lighter, it allows for proper adjustment of the afterlength, and for both those reasons usually gives better tone. It is slightly more expensive.
The customer went with the first route, the less expensive route. When I arrive at the music store, I see the violin, a cheap 60 year-old instrument with a tailpiece made out of some hardwood died black to look like ebony. The tailpiece is overly fat, nearly too thick to allow the fine-tuners to be installed, and too long to let the arms swing free. A real pain in the butt. Since I'm away from the shop, I have a limited selection of hand-tools. So, I'm modifying the tailpiece to allow me to install the fine-tuners, taking far longer than I had allowed for, with tools that I wouldn't use in the shop. Not a good way to make money. I did stick with my estimate, and the tuners are installed in working order, but it did further my commitment to not make estimates sight unseen.