It does not matter, but the maximum you can turn up the volume is at the least wattage speaker to not damaging it (providing you set the power output equally to all speakers).

In case of 70W speakers in rear, the max power from amp is 70Wmax. Anyway, in theory, you can adjust fader to balance front and rear but how to know the power is even 

TS how you define sound imaging? like the sound let you feel and imagine on how exact the environment of your recording?

your media is song or film?

but will it be wierd if the vocal is coming from the behind like the singer is singing behind you?

IMHO, i will only invest in rear speaker if its a rear sub or its for the leisure of my passenger.. for myself i will give my best buck to improve on the front sound stage.. with proper time alignment you can get more natural sound presentation which is coming from front.. like u r sitting in the concert.. unless u r playing movies u need surround sound in multichannel