Can you tell while you are riding your bike if you are one or two mph slower or faster now? I ride in pretty much constantly rolling terrain. My speed is never constant. My cadence constant.
For real time information your Garmin 130 will do well both with and without the sensors. It'll almost always be something in the log file you read later that may bug you. But when looking after the ride at a graph that is squeezing 30 plus miles of ride into eight inches of browser width, it sort of make you think something is wrong. Usually it's not.
For some every little discrepancy or even a jagged stepped graph bothers them. But for me a split second worth of discrepancy in the data won't keep me from doing what I need to do or realize what I did.