Hello Phil,
For all but the most exacting audiophile (who probably would not be doing multi-channel audio anyway

), the capacitor upgrades are probably overkill for the rear speakers. The added warmth/clarity/etc given by the upgraded capacitors will be mostly masked by all the additional soundwave reflections in the room (and your ear's ability to establish a reference).
The much harder decision is whether to use the Free Swinging Tweeter (which basically adds 25% to the cost). In my demo room, I use the older spiral ribbons in my surround 626Rs. However, I did this because I specifically wanted to be able to show customers the differences in the two tweeters. In two channel listening, the FST is absolutely better. However, in "real-world" rear channel listening, it is really, really hard for me to tell the difference. In most cases, I believe the cost/benefit of the FST in the surrounds is a toss-up. The one time that I would absolutely recommend the 626R with FST for the surrounds would be if you also are using 626R/FST for your fronts and center.
Julian
www.sedonaskysound.com