@David613, from what I understand, it depends on if it's an enquiry or a booking request.
For a booking request (where, once you hit the accept button, it's booked) you have to accept or decline within 24 hours or your response rate will go down. I have read on the forums that if you decline a lot of bookings, this can have a negative impact, but I'm not sure if this is true or what that impact is exactly.
For an enquiry, if you respond within 24 hours, whether that is to pre-approve or decline the booking, or simply message the guest, the 24 hour restriction is then gone and you shoudn't be penalised. If you don't do any of these things, I think your response rate will go down.
If I am waiting for the guest to respond to my message, i.e. I need more info from them, I will wait. But, if it's clear that I can't accept their booking, I prefer to decline (explaining why) so it's cleared from my dashboard. I don't think I have 'dismissed' one before.
More experienced hosts, please correct me if I'm wrong!