HOLD UP THERE FOR A SECOND. You are wrong, every print vendor I use specifies converting to CMYK, not RGB. Also, how in the world can you guarantee the colors to match your screen or printer? The only way to color match is to either provide a PMS color, or a hard copy of the color you expect to match.
CMYK is for PRINT and RGB is for SCREEN / WEB
Can someone else who has more experience in offset (and/or digital) printing please chime in on this? Please?
Wrong? Doubtful. I've been doing this longer than you've been on the planet. And probably your parents as well.
The thing you fail to comprehend, no surprise, is that virtually all systems used by commercial printers, and your very own RIP if you print, will do a far better job of converting the RGB image for printing than any software you might have.
How do I know this? It's the result of doing exactly what I advocate. For years. When I have a job to print on my own equipment [currently a Mutoh 1204] it do it in one of exactly two ways. Either I send it directly out of Flexi or I import a full size RGB jpg into Production Manager. There is no other way I will set up a print job. When I do this what I see is pretty much exactly what I get. Trust me, I've done it every way possible and this produces the best results by far.
Now then...I merely extrapolate this experience for print jobs I farm out. With exactly the same results. I realize that virtually all commercial printers specify CMYK. I tell them, each and every one, that if they want my business they'll deal with RPG jpg files. If they don't like that, there's plenty of printers that will accommodate my requirements.
As an example, I had a large print done on fabric for one of those curved 10' high exhibit backdrops. I was dealing with a big name outfit that should have known better. Anyway, I sent them a full size 150ppi RGB jpg. I told them to take a look at this file because that's exactly what I wanted the print to look like.
They called after a couple of days and said that they were sorry but when they tried to print this file the blues all shifted to a muddy purple and the other colors also experienced extreme shifts. This image was mostly brilliant blues. I asked if they converted the file I sent them to CMYK before they sent it to whatever they were using for a RIP. To which they replied "Of course, we have to do that to print it."
I told them exactly what I wanted them to to. I told them to send the file I sent without any modification to their RIP process, set the rendering intent for bitmaps to 'Perceptual', and print the damn thing. They said they would but they've never done that before that I would have to accept the outcome no matter what. I told them That If they sent the job EXACTLY as I told them to do so, that I would accept whatever the outcome would be.
A couple of days later they called. The kid was amazed that the print came out pretty much exactly like the file I sent them. She said that no one there had any idea that you could do that. I just smiled and remarked that I really should charge them for opening up a whole new world to them.
