Everything you said is correct. Including that it isn't really necessary to go to the trouble of adding the resistors to make a equivalent 8 Ohm load and still use the two speakers. On the other hand, it won't hurt anything if you would like to try and you want to spend the time & money. If you do add the resistors as you describe there will be two changes.
1) The acoustic power output will be attenuated so the amp will be perceived as just a little quieter.
2) The tone will change a little since the impedance of the resistors over the frequency band is fixed. That's different than a real speaker's impedance which varies quite a bit as the frequency changes.