How to know when you need an alignment on your car?

I have just had my back brakes done, front still at 37 % and tires rotated, along with a complete safety check and my mechanic says everything is great on my car. He took it for a test drive after the brake job 2 weeks ago and said everything seems great, but, I notice when braking on steep downhill grades going 50–65 mph that the car seems to “shudder” and the steering wheel vibrates. This is only when braking at high speeds on a downhill grade. Is this an alignment issue? My last alignment was over 2 years ago, but, my newer tires are wearing well and my car drives perfectly straight with no hands on the wheel for a long distance, at least a block or so. Whats your expert opinion?