First off the default rendering in X4 of automatic (Saturation for vectors and perceptual for images) was just awful. The default rendering of relative colorimetric in X5 in my opinion was another mistake.
I have attached a PDF of an article I wrote some time ago on Black Point Compensation contained therein are definitions of rendering intents and a reasoned argument against BPC.
First BPC is NOT ICC complaint and therefore should NOT be any standard PERIOD. Adobe uses this with relative colorimetric rendering to simulate perceptual rendering but in fact in my opinion simply induces another variable of no real use to our industry.
Second converting wide gamut color spaces such as RGB to narrow gamut color spaces such as CMYK is like putting a 500 lb. man into the suit of a 200 lb. man. THERE IS NOT RIGHT WAY!
The ICC definitions of the rendering intents are a good guide, I use perceptual rendering for any wide gamut to narrow gamut conversion because it PRESERVES the feel and PERCEPTION of the images color relationships. Color and how the viewer sees it is in fact a perception not an exact science.
I use relative colorimetric rendering for color conversions where the destination color space is equal to or larger than the source color space since this properly fits the definition of the rendering intent. Relative colorimetric conversion by definition is an EXACT SCIENCE.
I will also post this in the X4 forum, for those of you who consider color a serious endeavor I suggest mt two books for CorelDRAW color management at www.graphictechnology.com.