I've already realized the value of those class 5 worlds and specializing them for specific tasks.

Couple example from my game last night:
1) Colonized a class 7 world in a system. Nothing special about it actually but I planned to build more influence related buildings on it since it was smack dab in the middle of the galaxy. Also in the system was a class 5 planet. I eventually colonized it as well and it had a manufacturing bonus tile. So I decide to try to make this a ship building planet. So I setup 3 factories and a spaceport leaving one tile open for now. Holy cow! That little planet cranks out my top ship (the Lasershark Mark II) in just 4 turns. Even on Earth that ship takes 9 turns to build. So this little ship building class 5 world is building my whole fleet!

2) Same game, got to a class 15 world nearby. Also in the system an icy looking class 5 with rings. I pretty much ignored it for a long time. Finally sent a colony ship to it (playing on cakewalk to try things out thus the AI isn't spamming colony ships everywhere). Of the 5 usable tiles, 2 are research bonus, one an influence bonus and another is an approval bonus (don't need that). Can you say...Research planet! Man that thing is pumping out beakers like you wouldn't believe.

So don't pass up those class 4, 5 and 6 worlds. They are useful!