My understanding has always been that lab software cannot, in general, read profiles, and assumes everything to be sRGB. Thus, nothing is converted to sRGB at the front end. Files are converted to the printer profile on that basis. If the printer profile happens to contain colours outside of sRGB, as they often seem to do, these colours get clipped unless the lab sends the file to the printer with "no conversions". This service seems relatively rare (I'd ask DSCL about this if I were going to bother). One reason labs might ask you to "edit in sRGB" is so you can't preview/soft proof colours in the printer profile that are later clipped. Buried on DryCreek somewhere is a sentence (a few years old) that says "many versions of the Fuji Frontier PIC driver crash when given files with profiles embedded", but presumably labs would alert customers to such a problem.