I'd recommend converting all of your images to CMYK. Since the gamut is smaller, you may get some unpleaseant surprises if you wait for InD to convert from RGB at the last stage of the workflow. If you convert to CMYK yourself, you'll have a chance to make any adjustments to the color that may be needed.
A quick solution is to create a Photoshop droplet to automate the process of converting from RGB to CMYK.
I've been discussing this on the HOW design forum. I have always worked strictly in CMYK for print work, flightchecked to verify there were no RGBs, then output a CMYK PDF. But recently, after hearing from other designers who have begun working straight from RGBs, I did a poster job all in RGB, then created a CMYK PDF from InDesign. It came out beautifully.
The advantages I see are:
- ability to work with all the filters and adjustment layers in Photoshop (some aren't available from CMYK mode)
- rgb files are 1/4 smaller because of only 3 color channels instead of 4
- not having to worry about flightchecking for RGBs
Disadvantages I see are:
- you may have gamut surprises that you woudn't know about until it's already at the PDF stage, and no opportunity to correct them
- If you choose "CMYK" when you make your PDF, how does it deal with spot colors? It seems you'd have to use "Leave Colors Unchanged" in order to get the PDF to recognize the spot colors, in which case you'd have to change all your placed RGBs to CMYKs, correct?
Choosing CMYK will not convert spots to process. Spot colors will remain
spot colors.
Bob