I recommend deleting any estimates on tickets before refinement. The most common cases of this happening are old tickets that were previously refined and new tickets with an informal estimate.
The main reason for this is the anchoring effect will normally engage and people often end up voting for the same estimate. This happens because the mind to fixates on that value, using it for comparison purposes which makes it much more difficult to change. This means even with new information such as uncovering an unknown or other facilitation work, will lead to reduced change in estimates compared to a ticket without the anchoring effect.
The second reason is that occasionally the team will start trying to work out why a ticket was given a certain estimate (especially if it looks higher or lower than expected). This quite easily leads to scope changes within the ticket, to try and match what was there previously.
Therefore, by deleting the estimate beforehand you can avoid both the anchoring effect and reduce any potential scope creep.