All planets have value, but it's still best in general to ignore lower PQ planets until the first colony rush is over.  Of course, if they're all that's available or they're in a good strategic location, snag them.

Are you playing DL?  Even then, the lower class planets are good the snag, but perhaps later in the game.

In DA an TA, things are a tad different, but it's still a good idea to ignore the extremely low PQ planets until after the initial colony rush.  Even those PQ1 and PQ2 planets turn out to be PQ14 or 16 and sometime higher once you get the terraforming techs.

PQ11 or above gives an extra boost to local morale, so those are the best ones in the beginning.  PQ 8-10 are your basic planets and can get you going for pop growth and econ as well.

I pretty much do what the other two have said -- go and grab the better worlds first, and then come back and colonize the low-quality planets later on.  I only ever settle a crappy planet early on if it's in a key strategic location. 

One other thing to remember when colonizing - while its often useful to pick up those low PQ planets, it can be more effective to just culture-flip them if they're near another, decent world. The best example is the low-PQ planet that accompanies the homeworld of many races. Flipping that thing is easy as pie, and you save a colony ship by not colonizing it.

NIce thanks for the tips guys.

I'ma fan of leaving the lower class planets to the "other guys" and then culturally taking them over with help from an ISB and the higher pq planets i snagged.

cheers