As machtool and Gbent hav already noted it is the preferred method to have the leading edge of the tool slightly closer to the work piece. If you look at the Bridgeport manual for the alignment of the head, there is a note that reads,
"When indicating as in figure 3, it should be noted that the table is fitted to be slightly higher in the front, usually about .0005"."
The figure three that is referenced in the note is the tram in the "y" axis.
That being said, since I usually use my mill as a "jig bore" of sorts, I always try to tram the head as close to zero as I can so that all tools cut relative to each other as close as possible. This way when I switch from one boring tool to another I don't have to reindicate every time in order to hold positional tolerances.