it makes no difference.
When migrating from 2007 to 2010, the content db is migrated and so is the ui settings as it resides within the content db.... i think its just xml and xslt content. The only reason that i could see to change the UI would be for UI testing to make sure everything from 2007 to 2010 is properly migrated over (custom UI wise).
You should disregard the 2010 ui as there are some features in 2007 that dont exist in 2010 and in 2010 that dont exist in 2013 so you might have some errors on pages where the webparts failed as they dont exist. From a UI perspective it makes no difference as from 2007 to 2010 to 2013 as it will change on each upgrade and wasting your time ammending them!
like i said using 2010 is just the middle man to prepare the content database (layout of content within tables) for 2013. Its the content that i would be more worried about and the associations like alerts and workflows that should stay intact. The ui is interchangable and shouldnt cause any potential issues when fully migrated to 2013.
The biggest problem would be as noted the content, like checkout items and workflows showing pre checked out state (not updated) even tho looking in 2007 its uptodate!
