Yeah, I wouldn't worry too much about LOW humidity during flowering. I've grown in closets with no humidity control - so both high and low RH - and usually I just mist the girls with some water if it gets really dry in there, but they've grown well without too much attention to RH. I had a big prob later in flower with spider mites during my last grow, and I don't think the excessive heat (>85 deg F) and low (<30%) RH for a few weeks really helped the situation. That's why I have a small humidifier in my cab right now...but since it's so small and my ventilation runs 24/7, it's not making a huge difference and my RH is still at <40% most of the time (and sometimes under 30%). I'd say as long as you don't see any serious problems (like mites), you'll be OK with RH even in that range of 21-28% (though you'll probably have to water a bit more frequently with RH THAT low). You could always mist the girls once or twice and see if it makes any difference (in the plants and/or the %RH). As long as you have good airflow in your garden, I think any misting that you do will probably dry out fast enough that bud mold wouldn't ever become an issue. Def keep an eye out for it, though, if spraying the plants jacks the RH above 50% or so for a prolonged period of time.
Just my $0.02