Reviving and old thread...
If not critical, i think the 15's are just as important as the 10's and 5's are...
Many believe that the first two weeks of 15's dont do much in the way of adding muscle. Everyone thinks heavy, low reps are the key.
But remember, one of the main principals behind HST is progression. Beside the frequency principal, by doing the three rep progression, youre signaling your muscles to grow with ever increasing work load/stress, and i think this is one of the key fundamentals to hypertrophy.
By eliminating the 15's, you are basically removing 33% of the progression your muscles are subjected to.
Will your see 33% less muscle growth overall...? Whos to say...
Personally, i am always going to start with 15's in my cycles.