bob
It's better to have two hands than one glove.
For bitmaps I always send RGB files. Setting your Rendering Intents for bitmaps to 'Perceptual" and printing RGB bitmaps is about the closest to WYSIWYG that you'll ever achieve. On the other hand, vector components do better as CMYK especially if your Rendering Intents for everything but bitmaps are set to 'No Color Correction' or, failing that, 'Saturation'. Anecdote alert: Some years ago I sent an RGB file to a company to produce a large trade show background piece. They dutifully converted it to CMYK and returned the result. It was awful. I called them and they said that they had to convert the RGB info to CMYK since that was what they were printing. I told them to humor me, send the RGB file to their RIP. I told them I would accept the result regardless. They did so. The result was perfect.
A decent RIP will ultimately convert things to CMYK far better than any other software you have.
A decent RIP will ultimately convert things to CMYK far better than any other software you have.