Not true ; but that is what most people will tell you so. If Samsung wants to know whether you have flash cook roms there are ways they can find out. This concept of flashing back stock rom to " cheat" Samsung is only a fallacy and makes one feel good about flashing hoping to make good later on when comes to claims

If you are really worried and paranoid about warranty , than I suggest you dont root and flash. If you do, be prepared you lose warranty claims and be mentally prepared to pay. If the technician do not bother to check - and provides you free warranty services , consider it a bonus. I have been talking about this many times - say I am lucky ( 5 times aleady) - I always got it done free even though my phones are rooted/flash with cook rom . I never even bother to unroot or flash back stock rom when I sent in my phones