Optimization is "evil" if it causes:

• less clear code
• significantly more code
• less secure code
• wasted programmer time

In your case, it seems like a little programmer time was already spent, the code was not too complex (a guess from your comment that everyone on the team would be able to understand), and the code is a bit more future proof (being thread safe now, if I understood your description). Sounds like only a little evil. :)