Lets think about that board with a bow in it .... As is more than likely the
bow is not consistent throughout the length of the board. If the amount of
bow increases slightly, with both ends of the board in contact with the
fence. Where is the board forced to go? ... into the side of blade. We have
two options here. If the blade is thin enough and the increased bow is only
minor the blade will flex enough to absorb the extra side forces being
applied. If the blade is rigid or the increase in bow is considerable then
things start to jam up. Hopefully at this point your saw in a little
underpowered and the motor will start to stall, if not you can guess what
happens next. You must remember that a rip saw is designed to cut straight
stock. A jointer is used for truing stock. I so often see articles in
various woodworking forums about near misses and unfortunately sometimes
more serious injuries. Nine times out of ten the operator was trying to make
a machine do something it was not originally designed to do. Always remember
there is no such thing as an accident. 99.99% of 'accidents' are usually the
result of someone doing something wrong, whether it be a deliberate decision
to do it or simply a lack of concentration on the task at hand

"Leon" wrote in message
. ..

As Lwasserm has indicated, the longer the fence the less important it is
that the board be perfectly straight.