Does it matter in BiaB if you select a key of, say C, or use the relative minor key, Am?
I think it depends on the style. I don't know how accurately the BiaB 'brain' does this, but I know from writing styles that there are certain key specific patterns you can write that will turn up in the preferred situation. I've never tested them in the 'wrong key'.
My advice is your best bet is to analyze the song and use the proper key.
But then using the relative minor or major might just produce some interesting results, or not matter at all in your chosen style.
Insights and incites by Notes
I agree with Notes. I don’t know because I never tried to do it wrong.
If you are working in a key signature with several accidentals but you leave BIAB in the default key of C, you will have many ‘wrong’ choices of enharmonic notes. For example, where you think it should be an A# it might appear as Bb.
Because it uses relative minor rather than "true" minor, IMHO, it does not really matter (other than if you choose to input your chords numerically, rather than by name, of course).
This is the classical vs popular way of handing key sigs. Technically there are no minor keys but in the real world where most of us live there are tons of songs we consider to be minor. To their credit PG recognized this from the start and allows us to choose a minor key if we want. As a keyboard player for example who would consider House Of The Rising Sun to be in the key of C? Or Superstition in G? It's Am and Ebm.
Bob
In the StyleMaker options there is a way to assign a pattern to a "Roman Numeral Mask" (I through VII).
If your song is in C, but in BiaB the key signature is in Am, and if you assign a certain musical pattern to a I mask, the StyleMaker is saying that pattern is supposed to be played on a C chord which is not a I chord in Am but a VI chord. (Note: the mask does not designate upper and/or lower case Roman Numeral notes the app takes care of that.)
All MIDI styles don't use the "Roman Numeral Mask" and if you are playing a style that used it, you might not notice that that particular pattern isn't happening in the right place.
Also, I've had mixed results using that mask, sometimes I just can't get it to work anyway so I end up not using it in that style.
Insights and incites by Notes