you know those doors that prevent you from leaving the spawn area before it starts
that is where you draw the line
I actually get to leave my spawn to get to those other checkpoints and as such can actually play for more than a few seconds before dying
it doesn't matter if they're on the spawn - from a logical standpoint the most well traveled path of any tf2 map is the path that leads out of the spawn room and as such when you've got stickies sitting there you have the highest random chance of killing someone with them since people you know have to come out of spawn eventually
EDIT
Bleck - BLU Pyro