If the corner is just a little bit round then I get away with some careful folder work coupled with careful shaping with the folder. Depending on the cover material and board being covered sufficient re-structuring can be done to produce a smart ~ gently rounded ~ corner. I will prepare some useful pictures for here ~ for now the folder images must suffice [124761].
For greater radius corners a fair bit of care and patience ~ and some manipulation of the cover material ~ is required. Problems lie in eliminating the chunkiness of multiple overlaps without shredding the material and leaving bare patches. It can be done ~ keep an eye out for examples around the home or elsewhere. High end diaries often have neatly worked rounded corners ~ as do some leather-bound bibles.This really needs some pictures and discussion ~ and ability to pare down leather. More time...please....
I realize also that a note is needed along the line of 'Why round corners anyway?' There are good reasons in addition to that of displaying your expert craftwork. I will include a link here ~ when there is something to link to.