Tank position matters. There is some leeway but the more you move it from its optimum position the more you're going to see the effects of it in flight.
A properly set engine is not fully leaned. If it were then those uplines would lean the engine into overheating and probably quitting. The fact that the engine leans in uplines is evidence that changing the tank position changes how the engine runs.
If the tank is too high then when you fly inverted your engine will lean. If you set the engine to run at it's best lean setting inverted then the engine will be richer when the plane is upside-up.
If tank position didn't matter then you could set your engine for peak and it wouldn't change during the flight. What we're essentially talking about is fuel level, not tank position. A higher tank makes the fuel level higher and vice-versa.
Do yourself a favor and get the tank as close to its optimum location as is practical.
A lot of scale planes have been lost due to engine flame outs because the full scale plane didn't take model airplane fuel tank location into consideration and thus the tank wasn't anywhere close to where it should be.
Now all that said, you won't see a lot of difference if you only fly straight and level but even then the engine will change as fuel is used up and the level goes down.