It depends on when the tax year ends, but if based on the calendar tax year then it doesn't matter. If based on corporate or institutional tax year or budget year then it can make a difference... or not. That is in fact why I bought in 2017. I was under the impression that my 2016-2017 budget year ending July 2017 was the only time I would be able to buy, so I bought then. (However, it turns out I was wrong due to a change in my circumstances, so I could have waited until 2018, but oh well.)
As for benefit from the 2018 iMacor not, there are a number of factors to consider here:
1) Machine longevity. A hex-core low end model is going to outlast a quad-core model, all other things being equal.
2) Machine longevity part 2. A 2018 model may be supported by macOS longer than a 2017 model.
3) Machine cost. A hex core low end model will outperform a quad-core mid-end model, and will likely cost less than mid-end models do now. This is especially true in Europe, where re-jigging of the pricing due to exchange rates may further decrease pricing.
4) Machine cost part 2. If you wait the two months or whatever until the 2018 models are announced, the 2017 models will be discounted significantly in price.
5) Resale value. If it matters to you, the hex core models will retain value more than the quad-core models.
6) Machine noise for high performance. If you buy the top-spec 65 Watt model (i5-8600?), it will out-perform the current fastest 2017 i7-7700K in CPU-bound tasks. And it will do it relatively quietly. Meanwhile the i7-7700K hits vacuum cleaner mode pretty quickly. (I should know. I had an i7-7700K that I returned specifically because of the noise, and I'm not even a heavy user.)
7) Bragging rights.

"My e-peen is bigger than yours because I have hex-core!!!1"
8) As yet unknown features. For example, my prediction is that macOS 10.14 will bring 4K DRM'd streaming to the Mac, but only to machines that are 7th generation or later, to include the 2017 models or later. However, I could very well be wrong, and even if Apple does introduce 4K streaming, they could be asses and only allow it on 2018 models and later.
9) Education purchases. If you are an edu customer, now is not the time to buy. The time to buy is around July, and it's likely the new models will be out by then already. Why wait until July or whatever for an edu purchase? Because in many countries Apple has back to school sales, that add yet more value to the purchase.