i had this question too but mine pertained to planted tnk with co2. my tanks i have the spray bar goin up to cause surface ripples for aeration. but i was told that with co2 its best to point down. less out gassing that way. still will be interested to see othr opinions tho
There's no "one size fits all" answer to this question.
Much depends on your plant arrangement, size/growth, and hardscape.
The end goal is to not leave any dead spots in your tank and have enough flow in such a pattern that it draws debris into your filter intakes instead of allowing it to accumulate anywhere. It usually takes some trial and error to figure out what works best.
Feeding time is often the easiest way to see if you've got good flow; just watch where the flakes go.
You can point it anyway you like, as long as the direct flow is not effecting your plants too much to where they are laying flat on the ground when they're suppose to be growing straight up.
I actually don't like using a spray bar since it limits the rated flow rate, unless you have a custom made spray bar. I just use an elbow at the end of my tube (for ease of aiming flow) and mount it in the front left upper corner, with the flow of the water pointed perpendicular with the water level just barely sitting below the water surface (spraying to the right side of the tank) in order to obtain a small amount of surface agitation. Hope that made sense.
The key is to have sufficient flow throughout your entire tank, to get the needed nutrients/CO2 to all your plants.
That's a great ideal starting point... then you've got to factor in the hardscape and plants, how they may interfere with that current...
Also, I've found on tanks larger than 36" I personally rotate this flow direction 90 degrees, and aim the outputs across the top of my tank (back to front), down the front, and then up to the filter intakes...