Placeholders 'stuck' in transaction history

There's no reason to validate and no way it would have any affect on your situation; placeholders are not corruption.
The only way to get rid of a placeholder is to delete it.
Normally, when you have entered the historical transactions for a security that cause your actual holdings to match the number of shares in the placeholder, Quicken "hides" the placeholder. To delete a hidden placeholder you must unhide it; and doing that may be different depending on which version of Quicken you have. (Another good reason to specify that when you ask questions).
One way to unhide a placeholder is to delete a transaction that is part of its history ... or change a transaction that is part of its history by altering the number of shares. Once the history doesn't add up to the number of shares in the placeholder, the placeholder will become visible again.
Another way to unhide a placeholder - if you have a Quicken version newer than Q2005, is to check the Quicken "Investment Transactions" Preference, "Show hidden transactions."
You should be aware: once a Quicken investment transaction has been "linked" to a placeholder transaction, that "linked" transaction will never act normally - it will never affect the cash in the account - even if the placeholder is deleted. (At least for versions through Q2006; I do not have Q2007). In order to "fix" a transaction that has been linked to a placeholder, you must not only delete the placeholder, you must delete the linked transaction, then re-enter it.
Having said all that, you don't have to delete the placeholders. I don't know why seeing them in reports should be troubling; if all the other factors (#shares, cost, etc.) are correct, the placeholders should not be a problem.

Thanks for the info and advice John. (FYI, I am running Q06.)

Thank you for the advice on how to show the hidden entries.

Thanks SO MUCH for this tip. I've been dogged by "hidden" placeholder entries. Your suggestion to SHOW HIDDEN TRANSACTIONS in the Edit->Preferences->Investment Transactions dialog is a LIFE SAVER. Thanks so much!

"Investment Transactions" Preference, "Show hidden transactions." Worked perfectly, thanks

Derek, Thanks for pointing out how to get to the place to edit preferences. Going to Quicken to get an answer is a complete waste of time. Thanks again