You generally set your white balance for the key light that is falling on the subject. If the subject is light mostly by your flash, then set it to flash. If it is lit mostly by daylight, daylight. Best scenario is to color correct your flash to ambient with correction gels (not necessarily the same as ambient, depending on the effect you are going for).
High noon or sunset? Bright blue sky or some clouds or a lot of clouds? Is the ambient light your key or your fill or what? Does your vision of the photos allow for two different color temperatures in the key and fill? Think about that because it may mean you need to gel your flash to compensate.
In any case, I'd agree with the others who said to shoot RAW with an Auto white balance, but I would add the advice that you shoot a frame with a gray card once in a while during the shoot to sample off of in PP later to actually achieve proper white balance.
Outdoors, and outside the 'golden hours', I quite often put a full CTO gel on my speedlights and set the camera WB to tungsten.
That made the light on my subject essentially the same color temp as sunlight, but gave the background(ambient light) a soft, light blue tone that accented the skin tones nicely. Yellow and blue are complimenary colors.
You're shooting portraits. The key part is making sure the subject is lit with the correct colour temperature. The easiest way to do this is to give her a grey card to hold and either set a custom white balance in the camera by taking a picture of the greycard (in situ while, she's holding it, in the spot and lighting you'll be taking a photo of her), or just do a wider angle shot ignoring whitebalance and shootin RAW.
I go for the latter option. Every time the location / lighting changes the subjects get to hold the white balance card again for the first shot. Then when I'm in the post processing stage I look for the cards. I do a custom white balance from each card, and then select all photos in the groups between the cards and copy the white balance settings across. That keeps it situational, consistent, and accurate.