Don't put a lot of work into optimization unless you have a good reason to believe it's going to make a difference. Because each database server behaves differently, this probably requires some local testing to determine what makes sense in your particular context.

Try creating a script with the Benchmark module that compares the time required for 1000 updates of all columns, and 1000 updates of just one or two columns, and see if the difference is significant enough to make it worth your while.