Color space for opening profiling test images

Thanks to all who repled to my recent post regarding the value of using ProPhoto RGB vs. narrower gamut color spaces. Here are a group of related questions which resulted from my thinking about this a little more. If I am using a custom profiling service, how should I open the test image files it provides me? What color space should my editing or printing software be set to?  How can I know the native color space of the test file without asking?  If I don't know the test image color space, how do I be sure to not change the color assignments when I open it using my usual color space (whatever)? Does it matter if the native color space of the test image file is different from what I normally produce and want to print? Could some profiling problems be associated with these questions? Sorry for so many questions, but I suspect they are all really just one! Thanks.