This is a great subject and already I see lots of great answers. Personally, I quote at night- it is SO time consuming. I also call it a Repair Estimate these days which gives me the ability to ask for more if I hit problems. I use my iPhone if I have additional repairs I run into and snap/ send a picture to the decision maker as the job is taking place.
My repair estimates read: Parts and labor to....
That way, in the unlikely event I am able to beat my own optimistic quote the extra is mine :). Customer doesn't know how much is parts, how much is labor. Years ago on my road trip reports I would provide time on, arrival time, leave time and time off. Huge mistake and unnecessary. My experience is the customer is much more concerned about a QUALITY job at a perceived fair price. If you hit a snag, get your decision maker in the loop as soon as possible. His biggest concern is keeping his boss happy and that means he can explain what happened because he knows what happened.

That's how I feel. There is more work involved preparing an "estimate" than just taking a service call over the phone.
Look at the automotive industry, I think that they average person knows that they are being charged flat rate where a technician spends 2 hours repairing a car and they get charged for 5 hours. The big difference is that the customer can't see that only 2 hours was spent on his car.

Another thing I find odd is that on all shop jobs, no matter what they come in for, we charge each customer about 1.5 hours to do a thorough check out of the forklift truck. Then an estimate is prepared which could take another 1.5 hours in looking up the parts etc. and although the people involved in preparing the estimate probably make more than the technician who checked out the truck that work is at no charge.

i always tried to talk them out of a quote by letting them know that if it's quoted that's the price. if we come in under on the labor good for us if we come in over good for you and because of that we're going to quote higher than what it might take on average to do the job to cover minor deviations like "stripped bolts" (or more likely lost bolt or that wire terminal you broke). bottom line, you want a quote you are probably gonna pay more than with no quote. i always found that if you tell it like it is up front there are no hard feelings after.

i all ways put this is a close estimate and does not include any unforeseen worn items you never know what will come up and who else has been wrenching on it

I estimate time and materials, then quote "up to" 110% of that. Then I bill for what time I actually spend on the job.

Usually it's just what I expected, but sometimes the extra 10% gets eaten up by rusty parts that refuse to come apart or damage that I just couldn't see until everything was taken apart.

If everything goes wrong and I can see that it's going to be way over estimate, I stop work and explain the situation to the customer. Usually they give the go ahead to continue the work.

Mostly though, the job comes in under estimate and everyone's happy.

I find that there are just few customers that want a estimate before doing any kind of work. The other ones, that have an expensive repair, I let them know about what the cost will be. Very rarely does it come in under the estimate and if it goes over I usually eat some of the extra time. If it takes less hours, then just "hang out" and tinker with other things on the lift. I wouldn't leave and charge them more hours than you were actually there. A couple places I go makes visitors sign in and out. Customer may not have a clue what you are doing but they sure can figure out labor hours. Then there are the other type of customer that completely trusts you and doesn't pay attention to anything, including the invoices. "Just make my forklifts work so we can run this operation smoothly, we have other things to worry about" I'm very careful not to do anything to screw up that relationship.

When working road service I always broke my estimates down by parts and labor, not including sales tax or shipping cost and I let my customers know this. I always gave myself a little lee way on labor time just in case I ran into those irritating things like stripped screws, etc., If everything went right with no gremlins popping up then I would come in under the estimate. When it was time for the customer to sign my ticket I made a point of highlighting the labor and letting him/her know that I came in under the estimated cost of repair. They were usually very appreciative and I got repeat calls from them. As a matter of fact, quite a few (including the one I work for now) did away with the need for estimates altogether after awhile and just gave me a max/repair and no need for approval under the max. If you said it was going to take you from 8am-12pm to do the repair but left at 10:30am but still billed them for the full estimate until 12pm, aren't you setting yourself up for a possible lawsuit?