Generally speaking emission nebs are rich in Ha data so you'll probably find them quite red.
You wont see any full mergers of SII,Ha,OIII and R,G,B information as the channels would negatively impact each other. An example blend performed by Mr. Croman is at 15% opacity -
http://www.rc-astro.com/photo/id1173.html. This is a matter of personal opinion, but from an aesthetics perspective I think it does enrich the narrowband wavelengths. If you're a narrowband purist, you'd strongly disagree. Again, this is also suited to the target you're imaging.
I guess my point is you don't have to always follow the traditional narrow band palettes as Mr. Fleming shows in his NGC6888 image -
http://www.flemingastrophotography.com/ngc6888.html