ghost merging or normal merging

Well if it's just 2 walls together, interlocking would be a whole lot quicker.

 

I would say yes it is a waste. It would probably would take longer to "ghost" merge it. Plus, it may not be as accurate.

 

Depends. If you're making a map in the dunes where it's difficult to set up guides, then I would use ghost merging. If you were interlocking on flat floor, it'd probably be easier to set up guides.

Though I would use ghost merging for both. I think it's easier for everything.