It's always such a conundrum. I've tried to make a best guess and then added the other spelling as "other last name". With English last names I've chosen the most common English spelling with original immigrants and then choose the most common spelling for each descendant family even though that means that siblings will be different. For German last names, I've asked for help from our members who are more familiar with those names. I also try to look at the original documents and make a judgment about who recorded the name. Perhaps that is from my experience indexing where it's obvious that some people who recorded the records were barely literate while others were clearly familiar with the names in their communities.
