when i started boxing, i gave my toons different names, having in mind Pink Floyd, the band i love.
So one was Pink, other was Floyd. I decided to tribox and the other was Pompeii (the italian city, destroyed by the vulcano where Pink did a movie, a series of videoclips).

when i decided to 5box, i followed the strategy of similar naming, just changing the `^ above the i. i thought it was better to confuse.
now i know that, ur opponent will always know wich is ur main, so it pretty much has more to do with being funny and joke with noobs. depending on the font that ppl use for wow, some cant really see the difference on the nick, and automatically say im hacking lol.

I think anything will do. similar naming. alphabetical naming. random naming.
but the best choice for me, and i would do now if i could, would be as Vyndree sugested. but instead of a letter in the end, i would put it in the beggining, like Xzin.
being able to identify the char better, is a plus. for trades its nice. i do my trades based on party position of my addons, i can barely see the difference on my nicks