On a two player map, I just scout at 10 and then either 15 pool or 15 hatch based on their race/build. On four player maps, I just choose one of those to do blindly (15h with late scout is somewhat of a gamble vs zerg and toss) and then find the best path back to my normal game once I've scouted their race.
I imagine you're more interested in advice for PvR... it's hard. Safest is probably to open with a gateway at the top of your ramp (i.e. old fashioned PvZ); but of course this rules out forge expo.
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