I just got my cornrows redone today. It's important to ask the braider not to pull your hair into the braid from the very edge/baby hairs, or you may lose or get breakage there. I always have them leave out the very edges of my hair no matter how "messy" they may think it looks. I can alway slick it down w/ gel to make it look "neat".
I think that corn rows in the long run are easier on you hair/scalp IMHO, whereas individual braids can get too heavy and cause you hair to thin or break...
I wash my cornrows either weekly or bi-weekly by just rinsing. I don't scrub... I dilute gentle shampoo w/ water in a little cup and pour it over my head to saturate, then I let the water run through to get the suds out. As they get older I'm able to lift each braid a little to let the water/poo get under and around it. HTH