I'm not sure I see the problem at 8.0 mixed. Anyway, from what I've seen, a lot of 3.5-4.0 women return (pretty) big flat serves reasonably well (sometimes more often than the guy) if you give them a ball they can reach, especially on the backhand side. As previous posters already said, kickers and slices seem to work better against the female partner anyway, so I'm going for most result for least risk/effort here. If the lady can obviously handle the kickers and I need to throw in some flat serves for variety, then I don't think it's jerky at all to hit right the flat serve at them once in a while, especially if they're standing way behind the baseline. Now if they stand well in, then I don't know if I'd slam a body serve right at them, but I haven't really seen that happen on a first serve anyway.
Now obviously if you're playing for fun with someone at lower level (male or female) and you can see that they aren't handling the pace on your flat serve, what are you going to do? Obviously it's jerky to keep hitting the flat serve right at them.