I used Solo cups last year as an intermediate measure between the germ cups and moving to bigger pots to go outside. I didn't cut holes all the way up the sides, but did 4 pretty big holes in the bottoms - maybe 1/4" - that was plenty. When the plants are still that little, I'd be more concerned about the soil drying out too fast if you cut holes up the sides, as well, but that's just MO - I wouldn't want to have to water more frequently.