I want to know what could I use to have a stable 5V excitation voltage for my load cell(wheatstone bridge) which I could also use to have a stable 5V reference voltage for my arduino adc. Would a LM7805(positive voltage regulator) could be applicable for this or I need to use a precision programmable reference like a TL431?
Virtually "any old voltage" will do if the "any old voltage" is used for both bridge excitation AND ADC reference voltage. Basically you will get no errors theoretically. This is called a ratiometric measurement.