Can I pass on a good IPS monitor if I get Retina MBP

If you are not using it for colour critical work then it will be fine, it's not a wide gamut screen.

I would be more concerned about the actual spec you decide to buy as the RAM is soldered to the board & cannot ever be upgraded once you have purchased, I would defo recommend the 16GB Ram upgrade. If you can stretch go for the 512HD also.

At the moment Photoshop & Lightroom do not support the retina display so they do not utilise the full pixel advantage, images are drawn in pixel-doubled mode— each pixel results in 4 pixels on the screen. Not sure when adobe are going to address this?