[darkreever]

Considering the fact that there are literally million s upon million of Imperial worlds out there, is there truly "only war" in the grimdark future?

Technically no, there isn't just only war. But the Imperium is a galaxy spanning space empire beset on all sides and from within by a plethora of forces looking to expand their holdings, kill, cause chaos, consume, raid, and keep secrets hidden.

What would you say the actual percentage of worlds currently under siege or involved in major combat operations against one foe or the other?

Likely very low, considering that there are probably tens of thousands of worlds in the Imperium.

Is it actually possible for Joe Citizen to perhaps live out his entire life in 40k without ever even seeing the 1st hand effects of war.

Yes, the vast majority of Imperial citizens do just this.

Would this be more common than one thinks?

Looks so, since if every Imperial world was in the grips of some kind of major conflict or war then the Imperium would have ceased to be long ago.