Shimano R785 140mm options I'm looking at replacing my wheels on my Cannondale Synapse, and I can get a pretty good deal on a set of Renolyds 6 bolt hub wheels, but Shimano does not make a 6 bolt 140mm set of rotors. 

What I'm curious about is the suitability of some of the other manufacturer rotors (Avid, others ? ) and their compatibility with the 785's. I have some trepidation about using something other than Shimano rotors considering the importance of me being able to stop. 

Is this something I should worry about? 

Should I get the centerlock hubs so I can use their centerlock rotors (+600 bucks)? 

Thanks.
If you can afford it, centerlock hubs are a nice option.  The rotors are easier on/off and have less tendency to go out of true owing to the large stiff carrier (Shimano).  

If you don't want to pay for that convenience, I have used many other brands of rotors in the past, and apart for bends and break in periods not had any problems mixing manufacturers.  I've used Avid rotors with Hayes brakes and vice versa and just about every type with Formulas and not had problems.  After bending rotors beyond repair I've often had to buy cheap "in stock" rotors to replace them with and most often been pleasantly surprised when it all works out fine (until the next crash or incident).

In my opinion, the Centerlocks are well worth the upgrade money, but if that's not for you, just about any other brand of reputable rotor should be fine.  You can expect a normal short break in period as the pads wear to the new rotor.