Re: F=MA or E=MC2
It happens on every level. Importance of this fact is reflected by the required accuracy in measurement. Yes ammeter draws current changing the circuit resistance and hence total current in circuit, but is that factor or change too much to have significant effect on readings? It is negligible for most of the measurements.
For example we all know light travels in straight lines, but actually it does not. And since we are too slow to realized that and it is not affecting required accuracy or causing a significant change, therefore we continue to neglect bending of light waves due to gravity, and that too gravity of our bodies 