That is a rather hard question to answer. It certainly isn't a yes or no.
Grasses need phosphorous for cell growth (including roots), DNA and RNA and for the photosynthesis/carbohydrate production process. Grasses use potassium for cell wall turgor, important in roots for water retention and better heat and cold temperature hardiness. Nitrogen is important for photosynthesis and growth.
Most soils have sufficient amounts of P and K to maintain an established turf at a level acceptable to many.
Applying NPK to a new GROWING lawn floods the soil making the nutrients readily available to the grass. This is important and advantageous in the establishment of a healthy thick lawn.
Applying excessive amounts of K can result in over-thickening of cell walls. Excessive N can result in susceptibility to disease. and Excessive amounts of N and P will result in their leaching resulting in water contamination and environmental issues. Especially important for your well.
Rather than one large application, it is better to spoon feed the nutrients by making partial applications every couple of weeks. This makes the NPK available to the new grass while reducing the amount available for leaching.
If that is not enough to consider, as your concern is the roots, root growth slows at when ST reaches 65F and eventually halts at 77F (mid June for many parts of the country), So P &K applications will serve no purpose once those soil temps reach 70 F.
Advice: Do a half app or less now of the starter or a triple NPK (10-10-10- or 12-12-12) then use an organic fertilizer until the end of summer.

Good luck.

Edit: I assumed you seeded a cool season grass. Yes?

This post was edited by yardtractor1 on Fri, May 30, 14 at 22:55

Thanks for the reply, and yes cool season Scott's sun&shade mix. I was thinking of trying melogranite (sp?) Next as it's organic and would be better for the well. Was thinking at the 8 week mark. Like mid July.

Also, well is over 250' deep to the aquifer. Asked many of my neighbors up here who have fertilized for years without contamination issues. If this helps at all.

Many of the people that say "switch to Turf Builder" are people only paying attention to what they can see. Encouraging blade growth over root growth, and shallow/frequent watering is the biggest reason for burn-out in Summer, and burn-out in Summer equates to lawn failure in brand-new lawns. Stay the course and sacrifice a bit of top growth now as insurance that the lawn will survive summer.

It appears you are mowing high which has a two-fold advantage for you. 1. Taller top-growth corresponds to deeper rooting and 2. Taller grass shades the soil and helps the soil and neighboring blades retain moisture.
Grass clippings are a 4-1-2 fertilizer and contain 50-85% water. So mulch mowing has significant advantages.
I can't think of any reason to wait that long to put down an organic fertilizer. Organics are slow release (Milorganite is now 70% WIN). Once the necessary environmental conditions exist, It can take 3-4 weeks of micro-organism activity for them to release nutrients. The sooner you apply, the sooner you build up the soil biology necessary for their efficient decomposition.

I'm mowing at 4". I was told the same things you mention. So, you guys think I'd be better off doing the organic now?

First, a big thumbs up on the watering advice. That is going to have the most substantial influence on deep rooting.
Based on my previously stated rational, I think there is still benefit to be had by continuing spoon feeding some P&K to the young grass through June. (btw I meant to say mid July as the time soil temps in some locals can reach the 70sF). I'm going to defer to others (including andy10917 and dchall) regarding adding organics to the mix or waiting. My opinion is to start incorporating it now in partial apps until you are doing full apps in July and August.

I would switch to organics now because you really can't use chemical fertilizers on any northern lawn in the heat of summer. And this is new grass! And for the rest of your summer (life) I would continue with the organics, mowing high, and deep watering when you do water.