As per Vitalik’s post quite some time ago, I don’t recommend using the secp256r1 curves.
" The obvious question is this: where did the seed come from? Why was the seed not chosen to be some more innocent-looking number, like 15? In light of recent revelations regarding the US National Security Agency subverting cryptographic standards, an obvious concern is that the seed was somehow deliberately chosen in order to make the curve weak in some way that only the NSA knows. Thankfully, the wiggle room is not unlimited. Because of the properties of hash functions, the NSA could not have found one “weak” curve and then gone backward to determine the seed; rather, the only avenue of attack is to try different seeds until one turns out to generate a curve that is weak. If the NSA knows of an elliptic curve vulnerability that affects only one specific curve, the pseudorandom parameter generation process would prevent them from standardizing it. However, if they knew of a weakness in one in every billion curves, then the process offers no protection; for all we know, c49d360886e704936a6678e1139d26b7819f7e90 could have been the billionth seed that the National Institute for Standards in Technology tried."