Typically all but some smaller body parts where isolations may be used, e.g., concentration curls, tricep pushdowns, calf raises, etc., seem to respond better to higher reps. However, the vast majority of our routines consist principally of compound exercises.
ON BB curls, I go with 5's and will even go up to "cheat" curls after I hit my 5 RM. On DB curls I usually do not go below 8 but will go down to 5's on hammer curls. BB curls activate more overall muscles.
I see I do BB curls anyway. What about skullcrushers in your opinion?