using 2 centers as mains

I have a combined 2 channel/ht system and my mains are Heron i's driven by Rogue M-180's.I thought I would try saving some juice and tube life so I am waiting for the receipt of another matching center(Meadowlark Pestral).I figure I will set these up as my mains and go with the phantom center setting.
Make sense or am I doing something stupid?

If you use the Petrels vertically, they should be OK. I cannot say that it makes sense or is stupid since you are deciding on matters other than sound.
Will you get "Phantom Mono" sound?
I can't figure out why running 2 center speakers will use less tube life than using 2 other relatively high sensivity low reactance speakers. I guess that you are running a powered sub because center speakers don't play low octaves as a rule. I prefer the bass of the speakers too the bass of a sub. You can still spare your amp by having a high cut off for your sub.
I will be hooking the Petral's up as the mains, horizontal not vertical, and the reason I save on tube life is the tube amps/tube pre will only be used for 2 channel now. I have a Rotel that will be driving the Petral's and the rear's.I should have explained this before.(and I'm probably still not painting a clear picture)
Using them horizontally is decidedly sub-optimal. Your phantom center imaging will suffer.

I could try them Verical but this will depend on the WAF. I better start priming now, before the new one show's up.