But how do you spend the skill points before you get a level? Are they class for your past class or the one you are taking? Sure, if they are the same class there's no issue. But if our supposed 5th level character is all class X and at sixth level they switch to PrC Y, if you let them take their skills before the class is selected which list do they use? Do they use the list from the level they've already spent the skill points for or do you let them spend the skill points for the class they haven't even gained yet? Nevermind the argument of how many skillpoints they get if the amounts are different too!
Why should feats be any different? How can you gain a feat for a hit die you don't technically own yet? Sure, you know you are getting a die ... but it isn't yours yet. It's like opening gifts on Christmas Eve! [Fun, but technically not right!

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There are guidelines for a reason. It may not be the most gamebreaking decision in the world, but it is still against RAW. As a houserule it is viable, but not one I would use.