An "audible impact sound".... =P
Anyways, I guess it depends how hard to kneed your phone although I'd say if there's no visible damage you're fine. If you're constantly kneeing your phone, on the other hand, then that might be different (obviously). "Strain/stress" to me is something that's occurring consistently over a period of time, and in this case you hit it once.
While lots of people prefer their phone as is, I'd suggest investing some money in a good quality screen protector. It would be well worth it to have that fail safe in the event you do accidentally hit the screen. But to each their own!