The important thing to remember is that sometimes the projects aren't what they seem to be so try hard to get all the details from the client initially and reiterate back for clarity.
I once received a proposal for a project that was for gathering names and addresses of doctors. The proposal also indicated that she wanted a mail merge and printed letters. Well, that was fine. I quoted x amount of time. As the project evolved, the client needed the letters printed on her own stationary and there wasn't enough time to Fed Ex the paper to me so I had to send the file back to her electronically for her to print from her office. It took more time to make sure all the info was there and that she received the file to print on her own, plus include instructions for her to print and line up the heading on her stationary. Then the envelope file had to be done. So what I thought, (and the client thought) was a simple mail merge, print and mail project turned into something else as time progressed.
There was no problem with the increase in time as I kept the client informed along the way but it is not always so cut and dry. Things change and therefore estimated time can change. Keeping the client on top of the situation seems to thwart any problems with that, though.
So do overestimate to cover things that might be unexpected.
It's like getting braces...the orthodontist says you'll have them on for 2 years but really you need them only for 18 months. Then if there is a delay for whatever reason the added time over 18 months doesn't matter and then if you get them off under 2 years, the orthodontist looks pretty good to the smiling patient!
Janine