I personally change position of fingerings depending on the progression I am to play, and in some cases the horn I am using. As for both of my 4-valve flugelhorns, there is no finger hook on the bell as I did not want this to get in the way of the dexterity of using that fourth valve. Of interest, the Getzen came as a standard feature to NOT have a finger hook. The Kanstul had the finger hook standard, and I had to ask the Kanstul company to LEAVE THE HOOK OFF when I placed the order. They complied with the request and it has made me a very happy flugelhorn player.
As for my trumpets, I adjust finger position depending an the nuance I want to get out of a tune. Bebop gets the the "correct" details to keep the fingers rounded and directly over the buttons for the most rapid response and dexterity. However, for ballads I like to change the grip to place my dip joint over the keys as this allows me to slow and even when needed, half valve an attack of a note so as to get more "personality" out of a passage.