Marmalade said
for example, the allegro molto scales in Saint Saens violin sonata number 1 are a great thing to practice speed on, but when is it better to play a whole phrase of staccatto's on one bow direction, or change bow direction every note?

It's usually suitable to play flying staccato when the staccato notes don't need to have that much emphasis or if you want to do something different or impress someone.

Funny you mention the Saint Saens Violin sonata no. 1 which has a movement that suits the up bow staccato very well. Emphasis on the first eight note following sixteenth note slurred staccatos.  It's one of those pieces that are famous for flying staccato. Saint Saens also wrote the virtuoso piece, Introduction and Rondo Capriccioso, which has just a little bit of the flying staccato as well. The most famous flying staccato piece would be Grigoraş Dinicu's,"Hora Staccato" and the most famous performer of that piece is the very guy who transcribed it, Jascha Heifetz!!!