Opioids are cross-tolerant, mostly but not completely. There is some benefit in switching between say oxy and heroin; I don't think it's generally all that large though.
I've found that once or twice weekly opioid use will cause a small tolerance buildup (if you're doing enough to hit you hard, put you near the nod level) but isn't too bad. Anything more frequently and your tolerance will start to go up fast. And additionally I've noticed the drugs become much less euphoric when doing them at a high frequence (once day/two days), regardless of dose.