Ever OK to slow-play trips?

Hi all,

I know the proper play is normally to fast play your trips (especially in lower limits), but are there occasions when it's ok to sneak in a little more money to the turn with a slow play?

Examples would include a rainbow flop so there are no flush draws out there.

Last night I decided to slow play a flop of 998 rainbow (holding A9), J ended up coming and someone pushed all-in w/ QT. I was able to lay down my hand, but was still disgusted with myself for not fast-playing it like I do 95% of the time.

Thoughts?

If that 8 wasn't there (say, 993 rainbow), would it have been worth slow playing?