To better understand why/when/where to apply fertilizer, you'd be better to read on what the individual elements...
N..nitrogen, P. ..phosphurus.....and K..potossium (potash) does for plant life including lawns, trees, shrubs, annuals, perennials, and bulbs.
In a few short words, nitrogen greens things up, makes them grow and can interact with how phosphurus works...which is usually to produce strong rooting and encourages flowering where applicable.
Nitrogen can, however, cause the phosphurus to not produce flowering, the plant is being told to produce foliage at the expense of flowering. Potassium aids the plant to produce strong rooting and gives winter protection.
Then the other elements kick in..zinc, molybedunum..(sp), copper, lead, etc, etc....you can read more about them in the fertilzier article.

Far from not applying, nitrogen in spring advances the greening up and helps lawn grasses fill in bare spots; it promotes strong growth. Where flowering plants might be near such lawn, its best to hand spread the fertilizer so it doesn't act on the garden plants. This is not to say that fertilizer for flowering doesn't have nitrogen in it....it certainly does, but at a much reduce rate.
Look at the famous brand of fertilizer for flowering...Miracle Gro...15/30/15...the phosphurus is much higher and promotes the flowering and rooting.

Some people use the individual elements in their fertilizer...this is alright used for what they do...Ammonium nitrate has nothing but nitrogen...superphosphate, 0/20/0 and triple superphosphate 0/46/0 has nothing but phosphurus and potash fertilizer can also be bought in its only form too.
To not apply fertilizer on a lawn is inviting poor growth, disease entry, and otherwise a lousy lawn. Some might suggest you apply nothing but organic matter....and that is fine, where such can be injected into the lawn and the time interval to wait for it to work. Organic fertilizer must work its way through the soil and be the stuff that the lawn needs...N, P, K...and trace elements...
Inorganic fertilizer works immediately, over long periods of time and has the trace elements necessary for good soil health. Too much of any good thing however, can cause problems much the same as too much of the bad.
Read articles on 'fertilizer'...and read further articles on "compost" and understand better the why's before the 'whens'.

Neil, pre-emergent is not a weed killer, it is, in fact, a seed killer. It attempts to kill the seed that is dropped by the weed in fall to emerge in the spring. It doesn't kill well, it controls some and lets others come through.
It can then help the weed not grow much in strength and if given higher lawn growth, shade can help finish the job.
But, you notice, pre-emergent kills seeds....so any overlaying of grass seed in the fall will come to no end in the spring. This is why pre-emergent has to be applied at the correct time so as to not interfere with growing grass.
Weeds grow at the optimum time in May and June and that is a good time to put down a pre-emergent..to stop them from producing seeds. Another time is in the fall.

I'm surprised at the element strength you apply in spring....7.0.l...very low nitrogen, no phosphurus at all, and hardly worth speaking potash. Do read the articles I suggested and it will give you better understanding of the elements.

Neil,

Since you live in Zone 5a I think you applied the pre-emergent too early! By June you would be susceptible to crab grass especially if you had a wet spring.

I suggest you apply the pre-emergent in mid-April. The the areas where you had a bad infestation you could do a second application six weeks later.

You are doing the right thing by putting down the least amount of nitrogen in the early spring. I have used Lesco's 0-0-7 fertilizer with Dimension. This year I could not find it and used a Lesco product with nitrogen. My grass was growing like crazy in April and May. I had to do a lot more mowing. The bulk of the nitrogen should be put down in the fall.

Thanks folks! :)

goren,

I was told by Lesco that pre-emergent works this way assuming we've spread the Dimension: "When a seed germinates, the young seedling sucks nutrients from the area where it grew thereby sucking the product we spread. It's going to kill it". He also said that if there is already few weeds and we have not spread yet, applying it in the morning with dew is very effective since the product will dissolve somewhat and is going to be suck by the leaves.