Questionable slow play?

[DeeJ]

Now I'm not usually one for the slowplay with only TPTK, but here what do the panel think.. Players here in the hand are reasonable but not brilliant.

10/20 Party, full table 10 players

I have A [img]/images/graemlins/spade.gif[/img] Q [img]/images/graemlins/diamond.gif[/img] UTG.

Preflop: I raise, UTG+1 folds, UTG+2 cold calls, lots of folds, SB calls 1.5, BB folds.

Flop: 3 [img]/images/graemlins/heart.gif[/img] Q [img]/images/graemlins/heart.gif[/img] 7 [img]/images/graemlins/diamond.gif[/img] (7 SB)
check,check,check

Turn: 5 [img]/images/graemlins/club.gif[/img]
check,check,check

River: 9 [img]/images/graemlins/diamond.gif[/img]
check, I bet, UTG+2 calls, BB folds.

Final Pot: 5.5 BB

My reasons for the slowplay. Betting out with the Qxx flop is unlikely to get a caller unless someone else has a Queen. (15% chance?) and will only get raised by a hand which I'm behind (twopair or trips, 5%?). The chance of someone holding 2 hearts is small (5%, with 20% chance of hitting = 1% real risk of losing, 5% of missing bets?). Therefore with a 75% chance of taking the pot then and there, I decided to try to get someone to hit a lesser hand, hopefully with an Ace, or some other pair (Jack,Ten). Obviously risky as I don't like Kings or the board pairing. By waiting until the river I am more likely to get a call (or two) as people will not put me on AQ.

Does this stack up? Or is it rather too risky giving two free cards with only TPTK?