Spokes tend to break though fatigue (caused by inadequate tension) - rather than as a result of over-tensioning.
Personally I've not used a tensionometer - I just pluck the spokes like a guitar string in order to check that they are all even tension. If a wheel is true, but tensions are not even around the wheel - it will rapidly go out of true.
Because of the dishing the NDS will be a lower note than the DS on the rear. Personally I reckon the A above middle C (+ or - 1 semitone) is about right for the front and the rear DS if using stainless double butted spokes.
I've read lots of arguments for tensionometers... all very persuasive, but the simple fact is that my own hand built wheels, built by "ear", have consistently failed to either go out of true or to break spokes..... which is not something I can say for some of the factory wheels I've had the misfortune to come across.
In my experience, a tensionmeter is well worth the expense.
Despite getting my wheels perfectly true in the truing stand and plucking each spoke for the sound, I still broke spokes - 7 on different bikes in the space of 9 months.
I then got a tensionmeter and measured every spoke. Although they sounded the same, the actual tension varied too much.
Park Tools say a maximum 20% tension variance - some of mine were 30% plus. I undid every spoke and brought them within a 10% variance.
Despite being 17 stone and carrying two panniers, not broken any spokes since.
Having said that, my local bike shop wheelbuilder, builds by plucking and has done for 30 years. He has an excellent reputation so I guess it works if you're a dab hand.
