The main reason for putting them is the middle is too keep them "neutral" in the case of an excess of oil getting into the valve covers via a burned piston pressurizing the pan etc. The thinking is that under brakes (or the hit of the chute) the oil will be in the front of the cover and vice versa, it will be in the back under acceleration. That said, if you are not burning nitro, you can probably put them anywhere that suits your purpose, especially if you put a baffle over the hole on the inside.
Roo