OK, I hear you on that. Likely by mid-May you're not going to have any problems with weather. But the Pakistan part of the road will likely be in horrible condition so you might have an unexpectedly slow time of it past Gilgit. So hold off but when you do book, just make sure it's cancellable as long as you do it 24 hours in advance. Usually in China, you make the booking but actually pay at the hotel. In a few cases, the hotel site says otherwise.

I suspect you'll find that coming from Pakistan, you're going to need to spend the night in Tashkorgan before pushing onward. There should be enough vacancies around mid-May to walk in and there are a few places in town. I personally wouldn't spend a ton of money. If you're stuck for something in Kashgar and just want to book something, even a throwaway to make sure you have a roof over your head, then contact the Old Town Hostel and book something directly:
http://www.kashgaroldcity.hostel.com/micro/hostel.php

Even if you miss the date as a no-show or it's a non-refundable booking, you'd only be out less than USD 10. If you plan to be in Kashgar area for a few days, then waiting until you arrive there, then book for Urumqi and Turpan, should be a viable strategy. On my Silk Road trip (which was in a busier time than when you're going), I was pretty much calling or booking about 3-5 days in advance for the next place or two down the line.