Extra fuel

[Interloper2448]

So I am fairly new to KSP and many ships I built have some fuel left over from getting to orbit. I want to use the fuel then ditch the engine but I do not know if it is better to use a low orbit and maximize the oberth effect or if it is better to use a high orbit, evenly splitting my remaining delta v to get into a circular orbit for an injection burn to another celestial body. Assume already in a co-planar orbit.

[Red Iron Crown]

Welcome aboard!

Almost always better to do your burns from a lower orbit when possible.

Also: Moved to Gameplay Questions.

[qoonpooka]

AIUI (No doubt someone more experienced will come along) you'll want to use it in the same manner as you'd perform your ejection burn.  The problem with this is that your maneuver time is dependent upon the TWR, which will change - sometimes dramatically - when you hit the stack separator and ditch the now exhausted stage.  (You'll also now have extra space junk, but you may not care.)

I think KER, at least, is able to figure out the length of burn including a stage separation, but KSP can't even adjust for change in mass during long burns.  

tl;dr - use that extra fuel for your ejection burn but be ready for the burn time reported by KSP to be wildly incorrect. 

[Sresk]

I find that I like everything in roughly 100k orbits. Still low enough to use oberth well but high enough that I can do long burns reasonably well without pointing my nose at the ground.

[ForScience6686]

Set your altitude dependant on your twr.  A high twr craft will complete the burn quicker.  But a low twr craft will need more time, which means it will travel further in the orbit.  If too low and too long of a burn, you can end up dipping into the atmosphere as you try and hold the maneuver node.  

I often will use up the remaining fuel of the orbit stage.  This allows a greater margin of error dv wise for my travels.  Guessing at the burn time isn't too difficult.  If you messed up, well you still have the saved dv to correct.