Crop marks are really a relic left over from another age. In offset printing they serve a function, but in a digital printing environment where almost everything is done in an inline process they really have no relevance — none. As a matter of fact in most cases we have to actually remove them if they’re included on pages.
If your pages are in compliance with our technical specs then crop marks are completely unnecessary. And if your pages are not in compliance with our specs, then we can’t print them.
So send your pages in spec … and leave off the crop marks.