I would say it's a minor nuisance to do the subtraction or addition of 455 kHz, so just go for it.
A signal generator is equivalent to the local oscillator part of the tuner. The tuner also contains (at least) a "mixer" stage. The mixer multiplies the oscillator wave with the signal wave, converting the signal wave to IF frequency. In the usual AM radio, the local oscillator is tuned by the dial to a frequency that is above the desired signal frequency by 455 kHz, so the signal is converted to 455 kHz range for the IF to amplify.
For a very thorough discussion when you have the time, see wikipedia:
https://en.wikipedia.org/wiki/Superh...er%20frequency.