In our team we have stopped estimating product backlog items.
In our context I believe it's fine not to spend our valuable time estimating product backlog item. Because, there is no need to tell, where the team would be in two or three months. Also since team is working on small small features PO wants all of them. For us having estimate will not change any behavior, so I would say it's okay.
In my past projects we always estimated PB. An estimate represent the cost of an item and helped PO successfully prioritize the items. Also estimation helped us doing the rough long term projection on when team would be done with the product backlog items. We estimated product backlog items during our PbG ( Product backlog Grooming) meeting. We use to have PbG meetings 2 to 3 days before sprint planning meeting. This gave enough time to PO to get the backlog prioritized and ready for sprint planning meeting. At times we did adhoc Product backlog estimation based on PO's need.
It is nice to experience both side of spectrum.