I would always suggest going for very efficient speakers with high sensitivity for HT. One poor avr can't drive all those speakers. My personal cut off is 90db. If your front L&R speakers are in the region of 86 or 87db, add a 2 channel amp for the front two main speakers.
And as reachkalyan said, there are no thumb rules here. You will know whether an additional amp is required only when you put one in the chain, and hear the sound improved.
Iam running a 5.1.4 system with a modest marantz sr6011, with efficient qacoustics concept 40 speakers of 90db sensitivity. It was sounding good. No complains what so ever. And then I added a 2 channel amp of 160w per channel, and the sound really improved. Especially on music in stereo mode. So my amp has stayed stuck in my HT setup now