You’re right in how dimensions directly affect resolution. The only drawback of working with a file at 300 dpi is that everything else is scaled. You’ll have to make all your type 30-odd pt instead of 9 or 12. Rule lines will be 4 or 5pt instead of 0.5, etc. If you’re just eyeballing it, I guess it’s no big deal, but those large files can become unweildy.
Of course if you’re designing print documents in Sketch you’ve already committed a crime against nature, so might as well do what feels natural. :)