No this isn't true! You can run tires up to 28" without problems. The reason most people run 26" is that it keeps the gear ratio closer to stock form and still gets you the deep lugs. Think of it this way, the 27" tires give you 0.5 inches more clearance than the 26" tires. However, you increase the unsprung weight and make the "gear ratio" higher and therefore make the low range higher so when you go to accelerate from a stop it takes more gas to move both because of larger tire (gear ratio) and heaver tire.
With all that said, if you do lots of mudding and don't care about the acceleration and use low range more than high then get the 27" tires and Love Life!! I used to run 27" 589's and they were AWESOME in the mud!!! however, they were not the greatest for trail riding or speed runs hehe...its just a matter of picking what you want.
I like the 26" because its kind of like the best of both worlds for me as I do 25% mud 50% rocks and trails.
Enjoy your ride!! and welcome to the forum!