Will a pig always stand when she is in heat?

Yes. A way that AI techs (Artificial Insemination technicians) test to see if a sow or gilt is ready to be bred is to sit on their backs to see if they will stand still. If the sow or gilt stands still when pressure is put on her hips (imitating the weight of the boar's front when he mounts to breed her), then she's ready to be bred. Her vulva will also be swollen and red, which is an indicator that she's in heat.

Sows will have a period where they stand to get bred, but they also have a period during their heat cycle (which is before and after standing heat) where they will not. It can be quite noisy and a bit violent during this period. The sow or gilt will go after the boar and vice versa, and sometimes fights and squabbles can break out--and even if they are tame pet, you do NOT want to be in the way of such "courting" behaviour. But as time goes on, the sow/gilt will come to accept the boar and let him mount and breed her.