Runescape - im training range here. i need some help.?

so here's what i was thinking. im using void,and im 75 ranged. a good friend of mine told me to kill tzhaars. i thought that was a good idea, but it seems the more popular idea to kill yaks. just a little more expensive, so i was going to kill yaks with knives (what kind should i use?) until around 80-85 range. then from there i was going to range tzhaars untill anywhere from 95-99...i dont know if i should follow that plan, and at 95 should i still range tzhaars? should i range tzhaars in the first place? yaks?