Box trucking and irp registration apportioned plates

I want to start an expedited business on box trucks. Im in California i want to render services to the next 12 western regional states my medium duty truck is under 26k class c ive been doin my research and i have been calling my states office for irp registration and the sad part is that not even the people that work there have and explanation on if i should not get them and there handbook is not clear
The reason i ask is because i seen several people not get them and they run interstate but i also read that they can get audited for not having apportioned plates if anyone can give me real advice actual information i would appreciate you immensely