That's nonsense, especially when different price points are concerned, but even at the same price point: opening Ironmonger over Scout is better 100% of the time.
That's not true. First off, there are always edge cases for such things. No matter what cards you are talking about or what they do, you might want card A over card B because it will make your fairgrounds or horn of plenty better if you already had card B. In your particular example, if the top 4 cards of your draw deck are all victory cards, then you would much rather have a Scout in your hand than an Ironmonger most of the time. Especially if you have a Crossroads in hand. Ridiculous, I know, but brokoli is right, there are no absolutes like that.
The only move that is right 100% of the time is "if you know you have the most victory points, and you can end the game on your turn, you should do so."
Not counting strange tournament things where it could theoretically be better for you to lose this 1 game in the long run.