Today I put in some pioneer a series 6.5" in the back of my car. Tomorrow i'm planning on ordering the 4" models for the front. I'm running them of the 18w head unit. I have a sub already and was wondering if it would be worth it to buy something like 150hz bass blockers for the front speakers. I didn't know much about car audio when I went and bought my head unit and it doesn't have a high pass filter.

In that case your best bet would be to install some bass blockers, however IMHO if you tune things correctly you won't need them. I've done this scenario many times for folks who either didn't want to or didn't have the money to buy a HU with built in crossovers or put an amp on the highs with its own crossovers. I idea being that you tune your Bass and Treble settings, or EQ, so that you can get the best sounding lowest distortion output from the highs. Then simply turn the gain up on the subs to level match with the highs and bring in the bass that you need. More or less.

Or just order a set of 40-50 uf caps. Place them in series with the + terminals. The caps in the bass blockers are electrolytic junk.... Buy some polypropylene if you can...