Will Photo Labs print with the white borders equally (top and bottom, left and right) or do they tend to want to print crowding left-top sides? I sure this is also another obvious question, but inquiring minds want to know. Ha!
I'd say no, but if a few might, at least it would take very special arrangements. In general, what happens if you send an oddly shaped image for the paper size you request, they print it so one side fills, and the other dimension cannot fit evenly. There's two way to do that.
The norm is that they will automatically fill one side and let the other side overfill, it runs over (paper crops it), so there's no white space. The paper is filled, you get the size print you requested, but the image is cropped, probably in an unexpected way (you surely would have cropped it yourself if you expected it).
There are some (mpix.com is my experience) that you can specify (I forget their words for it, something like exact size) so that the other dimension fits and fills, and then one side underfills, leaving some white space along that side that you can trim yourself later. This gives you your full frame (that you sent) without any cropping.
Printing at home, your print menu probably does easily offer the option to center a smaller print, etc. And I suppose it is not impossible that theirs might too, but surely it will take some very special arrangements, special handling. It is not in my experience.
It has always seemed convenient to me to accept the realities though, and conform.