If you are turning in HQ items on your triple turn-in leves, then you are getting a 600% experience increase, and no single turn-in leve gives you six times as many experience points (it would only be 200% if you turned in HQ items, and the exp rewards are not anywhere near that difference). The HQ turn-ins are what make the triple turn-in leves so valuable. If you are not confident in your abilities to regularly HQ the items that you need, then, yes, stick to singles. When you can HQ with regular success, then switch to the triples.