meh. I lift in Chucks for nearly everything. For squats (I'm at 305, which is about 1.5x bodyweight), I do use weightlifting shoes to position my knees better, but if I"m not doing it that heavy, I may just use my Chucks.
You can get by without shoes for a good long time. At some point, you may find it useful. Rippetoe recommends them, but YMMV. I've found that my groin muscles are less likely to get tweaked if I wear them when lifting heavy. But if you're doing 20 reps, you don't need them. I'd cut down on the reps, though, and up the intensity. 3 sets of 5 reps at a higher intensity is better than 20 reps.