Physically realistic as throttle and stick take you low without problem.
As far as how the plane are actually employed in real life, it's not.
I'm reading a book about F15E during the gulf war, and the F15C providing cover to them are flying higher (at least 20,000ft IIRC)
But then again every one flies the way they want, I'm not saying everyone should fly high.
I fly Mig 21 mostly, and low flying ppl are easier to sneak on and kill.
High flying jets are a nightmare to get to without dying.
So by all means, keep flying low people haha. :P