Someone that works for the big 3 (Experian, Equifax, Transunion) would be best to ask, but my guesstimate: Asking for a copy of the report isn't going to hurt your credit rating any further (most people are already going to take a hit for the credit check and for opening a new account)
My understandings:
Things that are 'negatives' -- not being in "good standing" with your current credit cards, opening alot of accounts (particularly if they are unnecessary -- hint: if you're opening this account just to get the 150 and run, you're going to be taking a big, short term, hit to your credit rating... hopefully you aren't planning to buy a house or get a car loan soon ), making alot of major transfers (shuffling your bills from one card to the next), carrying too much debt for your income
To a lesser extent: Credit card companies running checks on you so they can send you different offers (not as bad as actually taking the offers and these checks disappear after a year or two, I think), not using your cards (if you have a card with $5000 limit and don't use any of it, you're probably better off closing the account unless you plan to use it in the near future), closing accounts (not as bad as leaving an account open usually)
'Positives': Paying off your balances, to some extent carrying a reasonable balance (but paying more than the minimum), having only as many cards/account as you need.
Anyway, that's my take: If you're going to open the account anyway, doesn't hurt you any further to get a copy of the report if they're willing to provide it. I'd wager to say the report won't be the "full" report you can get online, plus they may run a different type of report (a local bank did a check with ChexSystems, for example).
hifive to ddjkdg for the link... I think I actually covered most of it (except for length of time) without reading the article *flex*