JVM performance with respect to uncontested locks has become much better over the years, especially so with
Java 6. It's now at a point where I would not worry about it unless you're dealing with highly contested locks (at which point you may want to abandon simple synchronized methods and blocks, and use some of the more advanced concurrency capabilities. If you do that, work through one of the Java concurrency books first, either Oaks/Wong or Brian Goetz.)
Are you facing an actual problem that prompts you to ask about this, or is it merely a precaution?