IS it crucial as to where the checker is placed in the aquarium as long as you can see it? Is it more effective if placed down where the majority of plants are?

If you have good water circulation throughout the tank, with no dead spots at all, I don't see where it matters where you put the drop checker. But, if your circulation isn't that good it seems to me that you would want it where the CO2 concentration is likely to be the highest, to avoid overdoing the CO2 and harming the fish. The reverse argument is pretty good too - put it where the most plants are to make sure you have enough CO2 there. Toss a coin and decide that way?