It's not unusual for a wheel to go out of balance at some point during the tire's life, but it's not related to rotation.
Unless you knock a weight off while rotating them...
Or, it was never balanced in the first place and you couldn't feel it since it was on one of the rear wheels. Now that it's on the front, you can feel it out of balance.
It's very common for a tire shop to spot a wobbly rim or a tire that won't balance right and stick it on the rear axle where it won't be felt. Then at first rotation time things actually get worse.
If it's not broke, don't fix it. I have seen many tires that were never rebalanced and didn't vibrate their whole life. If it runs smooth, WHY give someone a chance and screw it up, and have to do it all over again?