I fly a lot, and spends weeks and weeks watching prices (and not see them change). And I used to work for online travel agencies. No, this is not true. Every person who claims it is true says "I watched the prices, and waited, then when I bought the ticket it was more expensive." Completely ignoring the fact that ticket prices generally rise as you approach the time of the flight. I've seen no evidence that the person would have paid less if they used incognito mode. I've also never seen a different (well higher price) if I signed in.