Shock unit in single rank (all engage) is quite effective against spear unit that is holding formation (not all engage), but should not use against other shock or unit that is engage at will. I use 2 or 1 and a half ranks against spears.

For me, any unit that has good charge bonus (3+), I will almost always charge them in wedge formation. Sometimes, the extra kill that I make in the first impact will help tip the balance. But I will never make a frontal charge at a spear/pike unit (too close to suicidal).