How flexible is "a dozen"?
Dozen is quite flexible when it is pluralized. While hundreds sounds natural, you don't often hear about there being tens of something. Saying there were dozens of something fills this void without implying an exact multiple of 12.
A dozen is a very specific measurement. It means twelve. If you have 11 items you can't call that a dozen, you'd have to say "around a dozen" or "about a dozen"; if you have 13 items you can say the same, or just call it a "baker's dozen."
Twelve is a small enough number that you don't really need much rounding.