Is it best to convert image to CMYK profile if printer has specified that they will print using a CMYK profile and they tell you what one that is. I've read conflicting reports about whether you should provide your prints with a CMYK based profile, sRGB profile or photoPro profile.
But it seems to me if your printer company has told you that it uses a particular CMYK profile (such as FOGRA27 Coated that my print company uses) shouldn't you always use that, and if they haven't, shoudn't you at least use PhotoProRGB instead of sRGB?
For example if you are working on a file in Photoshop and you do a SoftProof to sRGB and it shows lots of out of gamut colours, then you do a soft proof to the particular CMYK profile and it shows less and different out of gamaut colours (I have plenty of files like this). If you now export the file as sRGB its going to have to clip/modify those colours that were invalid for sRGB even if they are fine for CMYK. Then when the printer company converts the sRGB profile to a CMYK profile it has already has some incorrect colours and the problem will worsen when you then convert to CMYK to deal with the colours that the CMYK cannot handle.
These colours may not even be the colours that were shown out of gamout when you SoftProofed to CMYK initially because the conversion to sRGB will have converted the colours that sRGB couldn't handle so when map back to LAB the colours will not be the same as the original image. You could get round this by modifying the image so it doesn't show any out of gamout colours but you would have to modify it so it doesn't have out of gamout for not just the CMYK profile but also the sRGB profile even though the sRGB profile is not being used for printing!
Additionally saving with a CMYK profile will allow you to decide whether to use Perceptual or Relative conversion, you don't have this control if the printer company does the conversion.
Alternatively if they haven't told you how they would print by you can softproof to sRGB and make some adjustments to improve the chances of a good print. But wouldn't it be better to embed the a ProPhotoRGB as this will always better represent the colours in the original image allowing the printer company to do a better job of converting the print to whatever method they use. The only disadvantage to this method is you would have to export as a 16bit tif rather than 8bit jpeg because 8bits is not enough bytes to properly represent the PhotoPro colour profile.