First off- Services run on the UI thread. So a service won't fix the problem of blocking the main thread. Of course getting a sensor reading is quick, so you shouldn't need to worry about it.
What else are you using the data for? If its only to update the UI, no need for a service. Just register for sensor events in your activity and update the UI when you get a sensor event. If you want to do something else, like save the data even when the app is in the background, then you may want to look at Services.