False.  The original reason for that little "rule" was that GIs would take apart their mags to clean them and reassemble them with the follower incorrectly attached to the spring.  This would cause the mag to bind when loaded with more than 18 rounds, resulting in stoppages.  Once this was figured out, a directive went out for GIs not to remove their followers from the springs when cleaning.  This didn't solve the entire problem, as you can still swing the follower 180 degrees on the little loop (see the pic) and you'll have the problem.

Ultimately, the spring attachment method was modified with the 30s to prevent this from happening.  The 18-round myth lives on today, despite the fact that the reason for it in the first place has long been resolved.

-Troy