As a habit, I typically setup all my instument tracks as stereo, because most of the instruments I use in Reason are stereo. Ultimately it depends on the intrument in Reason. Reason defaults to left and right outs on each intrument, but it is not required and if you are running low on track space, think about using mono for som instruments like bass, solo strings, etc.
When I setup the PT session, typically I have already been working in Reason and am nearly done, I use PT to record live instr. and vocals and mixing. I will set up a number of stereo instrument tracks to match what I need for Reason plus 2 more stereo just in case I want to add something on the fly.
I don't know if this is the best way or not, but I remove the mixer from my Reason project, and then I plug each instr. into the hardware out seperately. Then I add Reason as an insert on the first instr. track and select the correct channel from the window that pops up, and then I just copy the insert to each track, changing the channel on each one.