Last week's Nickname-Rich Name of the Week was Catherine and her various spellings. I took a look at the U.S. stats to see how the various spellings have fared throughout the years. In addition to the three spellings in the search I performed, other spellings like Katharine appear, but I did not include them since they occur in much smaller numbers.
Kathryn, the least "traditional" spelling of the trio, has never been the top spelling but has gone up and down with the flow of the other two spellings. Catherine was the most common spelling until the early 1970s, when Katherine took the spotlight. What is interesting is that while overall all three spellings tend to follow each other in popularity, Catherine became unfavorable in the 1980s (when it started to bring in less than half of the number of Katherines) when the other spellings saw an increase. I personally think that's a plus for the Catherine spelling, as it makes this version the less expected and more old-fashioned-feeling one.
If you have any thoughts on this subject feel free to post a comment.