Yes. I beleive you. I just think that it is less risky to SLI exact models from the same manufacturer. Again, "ideally" is my nitpick, and I reiterate the phrase "minimize the possibility."
"For example, it is rather difficult to get two Nvidia cards obtained from different vendors, which deviate from the reference specification, to cooperate in SLI. From a pool of six different GeForce GTX 580 cards, only two would work together consistently. From a pool of five different GeForce GTX 570s, only three would. We had problems mixing clock rates, BIOSes, supporting platforms, and so on."
http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995-15.html
In fact, ASRock's Extreme 4 owner's manual warns that exact models be used in SLI.