I would say that depends on a variety of factors:
First, I find it necessary to ask, in what manner, if at all, is the offer being presented to you?
Is the person ready and willing to pay up, without a ‘reminder?’
If you have to demand it, I’m not sure it’s worth it.
1. Did you go completely out of your way to unite the pet with its owner? For example, did you chase doggy around for hours, missing the last half of your work day?
2. What is the financial situation of the owner? Obviously if they made the offer they must, somehow, be able to pay, but how much of a hardship will it be on the person?
3. How much do you really need the money?
4. Are there any reward alternatives? Permission to visit cute doggy every now and then? Date with foxy pet owner?
Personally, I doubt I would take the reward, however, I do think the finder is entitled to it without any guilt attached.