I've read-and experienced, that it depends on the level of the activity as to which way your BS goes.
When I do very extreme exercise (e.g. a spinning class), my BS ALWAYS goes up. It can sometimes stay up for quiet some time too. If I'm doing a more moderate class (e.g. body pump), I usually have to take dextrose half way through because I've dropped.
Overall though, the exercise is lowering my overall readings and making them steadier.
As for what readings should be before exercise, I will do the exercise anyway. Obviously, if it's low (below 4) I wont, but that is the only exception. I'm not advocating that everyone should do this!
I also think there is a lack of research into this area. Has anyone read any papers on the effects of exercise with diabetes? I mean the physiological aspects-not just 'yes do it, it's good for control'. I also am pretty sure that these effects will be very different for T1 versus T2.