So you can see your knob, and grab a hold of it without also grabbing a clump of your pubic thatch! lol!
Seriously though, depends on who you are, as to what you can get away with. If you have steady hands, use scissors, but be very careful... Trim the main thatch back, so it looks something in proportion. Hard to explain... Like, I have quite long head hair, so I can arguably get away with longer hair downstairs too (also having no gf heh) Whatever you do, don't forget secondary areas i.e. ballbag, and the areas where your legs join the bottom of your torso.