Ideally you should overclock just to the point where it's stable and without HW errors. It's not just about getting the maximum possible hashrate out of your cards. Stability and power consumption are also a factor. Typically the more more you overclock, the more power is used, so there is a diminishing return point where overclocking is just pushing the cards over their capacity for little to no benefit in your bottom line.

From the calculations i've done it's more profitable for me to get a higher hashrate rather than balance the power usage out, however stability is still important. As mentioned above with 2075 mem freq I get 29.6 MH on ETH and 860 MH on DRC at 150W which I think is reasonable enough aslong as those 1-5 errors per 30 min isn't an issue