Any sample would need to be random. However, we can assume that people may come to the bagel shop at all times it is open, and their opinions would be different from others. So, we should take a stratified sample of each hour the bagel shop is open. For example, we would randomly select a number of customers from 8 - 9, say, and continue that process for each hour the bagel shop is open.
Hope this helps!! :)
