The other thing against 15,12,10,8,5,3 is that it would be a lot more bother working out maxes for all the rep ranges.
Don't forget that it's all about progressing the load from start to finish. If you do negatives after the 5s you could be progressing the load past your 1RM! So there'll be plenty of progressive loading from the weight you used in the 15s to the end of the -5s with 15,15,10,10,5,5.
Entirely, up to you though. If you give it a try, let us know how you get on. I love HST