Something that would make customers' life a lot easier would be the existence of rules on the front-end. Sure you have the small squares that give you a hand but let's say you have a poster and want to make equal tiles or so? Either the customer has image manipulation skills and software (like photoshop) or the customer will have a really hard time doing it.
This will make the tool more easy to use for the people who spend money on the site :)
I agree with this. This goes along with the "grid mode" feature some people have been asking for. A more precise way to know real-life dimension is essential.