It's usually not a good idea to grab the "Mars" planet in your home system - since it's right next to your homeworld, when the AI eventually colonizes it, it'll flip pretty quickly back to you.

As to whether it's better in general to snag low-PQ planets immediately or wait for something better, that really depends on the circumstances.

Neat trick, Zarnick!

Since I put a lot of resources into early research, I do not make a new colony ship for several turns.  That leaves me with only the one you start with and the converted miner.  I generally send the flagship in one direction and the colony ships in two others.  If the flagship finds a high PQ one that I cannot pass up, I shift production gears then.  Similarly, if a gree resource is spotted, I shift to building a constructor.

I covet early green resources so much that I have sometimes sacrificed one of the early colony ships by converting it into a constructor at the resource location.