No, they actually don't 'require' a heavier gauge string. This has been a long told myth perpetuated by guitar magazines when more bands started using lower tunings. It was always 'suggested' that you use a heavier gauge string if you didn't want floppy strings. One big reason for this is the assumption that you only have maybe one or two guitars and you probably keep it strung to standard E, so tuning down a whole step or a two might cause some floppiness and muddiness and if you were only tuning down because you wanted to learn like one song that happened to be tuned low you would experience this. I can only speak for myself and play rhythm and lead using 12 gauge strings because that's what I grew up using, but my playing in general is 'light to the touch' it only looks like I'm playing hard and violent. A proper setup will disable you from needing to use heavy gauge strings is my point and any decent tech will tell you this, especially if a guitar is going to be dedicated to living with a particular tuning for awhile.