Usually leave box checked unless the merged name is identical to the merge target’s name.
When merging performers, there is a checkbox that says, “Set performance aliases to old name.” The performance alias is used to show what name the studio used for that performer on that particular scene at the time of release. When this box is checked, all scenes without a performance alias will fill this field with the pre-merge performer name. Typically, the box should only be unchecked (it’s enabled by default) when a merged name is exactly the same as the merge target’s name. We don’t want StashDB to say “Jane Doe (credited as Jane Doe)” for example. This will sometimes require separating merges into two submissions: one with the box checked and one with it unchecked.