I notice when I do tricep pushdowns that my abs get engaged. Also, looking in the mirror at my form as I do reps, I tend to hunch over the V-bar, with shoulders forward as opposed to standing perfectly straight with shoulders back. Can someone clarify the correct posture?
 
Also, when nearing failure on preacher curls (which I find comes very suddenly!), is it acceptable to 'cheat' in order to squeeze out another rep or two, or should I just keep my torso relaxed and once my arm stops working, accept defeat at that point?
 
Cheers,
Ian.