So my advice on common IOs at least would be to grab some as soon as you can at level 10, then maybe upgrade every 10 levels after that, so that you can feel the value of the incremental improvements, but not be wasting tons of money on enhancements, like you would be if having to replace TOs/DOs/SOs.
I recently used Merit Rewards to purchase SO's at lvl 20. They help a lot. And I don't waste Inf
Yeah but could have used Merit's to make influence to buy IO's instead. In the long term your gonna spend far more keeping SO's viable then you would if you invest in IO's up front.