The description is that you set the same in softproofing and the printer. But what if you are not printing but sending to a "make your own post card" service? Is there a general rule of thumb you can use to get a good indication - an average print profile coupled with "they generally print "perceptual"?
Seems in discussions that perceptual is the default ICC complaint version. But then I found a guide from a printing service, saying they use both depending on the medium:
"American Frame uses the Relative Colorimetric rendering when printing photographic images on glossy papers such as Epson Premium Luster Photo Paper to preserve color accuracy. When printing to canvas and matte surface papers, the Perceptual intent is utilized."
So it seems that unless otherwise noted, I should proof it for perceptual (which often changes the image most), while if the service does tell otherwise, I can proof it for relative.
Since this is all about getting out of gamut colours in gamut (smooth mapping vs cropping), the best alternative is to make sure they already are.
I am not an expert in this field, but given that one of the best online printing shops in Germany tells you explicitly what intent to use when soft-proofing (perceptual or relative) using their ICC profiles, I guess the optimal choice cannot be separated from the paper choice and from the rest of the printing equipment.
Ask the producer of the ICC profile you are using, you cannot guess properly otherwise.
Check here as example: http://www.saal-digital.de/service/icc-profil/