As you know, I've not seen the last two so this is with a great deal of speculation;

If I had to venture an opinion, I'd say viewing order doesn't really matter as long as the films are viewed in two sets of three. Preferably with some time to ponder between the two, maybe a week or two.

The trilogies look and feel completely different. They were created differently (live action vs CGI, film vs digital), have completely different visual styles, different story lines, different actors, take place in different time periods, etc.

It's Lucas' continued attempts to try and make them a single 12-hour film that has created so many headaches and fractured the fan base. All six don't work as a single entity and no amount of rearranging, CGI, reediting, or color alteration is going to solve that.

The TRNers are never happy trying to reconcile plot points that don't exist, nor are they happy with the live action & model effects of the 70s. The original fans are never happy with the CGI, or the lack of story from the 90s. The fan base seems to do better when they're allowed to go to their neutral corners, their decade in tow, and claim victory. The fans don't want to play together anymore than the two trilogies do.