Depends. If the opposing forces originate in different solar systems, they'd most likely meet in deep space.
Actually, wouldn't they need to be VERY (un)lucky to actually meet in deep space? -considering how big the place is- unless, of course, this happens on a (more-or-less) direct line between two specific points that are part of the conflict.
The best way to defend your planet is to intercept attackers as far away as possible. If a battle does take place inside a solar system, it'd happen at or near the target planet.
If indeed the target
is a planet; could be an asteroid, a comet, some artificial structure...
Or it could happen as far away from that target as the defenders are able to come to intercept the intruders.
Iain M. Banks has a great armada coming through space towards their target-system in 'The Algebraist', once the attacking fleet has reached the halfway point between their point of origin and the system they want to attack they turn around their ships and use the engines for deceleration and the sky lights up:
There's no such thing as FTL flight in this novel so, amongst other things, the space battle parts are a great read
An attacker would have no reason to come close to any other planets, as they can approach from a plane other than that on which the planets orbit.
Unless they want to hide behind them
I wouldn't expect any dogfighting. Missiles would be launched from extreme range by whichever ship/fleet was able to get a visual first.
Right, dogfighting would only happen in the sky above a planet, not anywhere in 'real space', missiles (and perhaps particle beams) is what makes sense to use in space.
Real interstellar war would probably be pretty boring to watch.
And the stress of waiting for years, months, weeks is the real battle 'cause the fighting might be over very quick.
The Algebraist, I can recommend it highly! -It's quite hard SciFi when it comes to the actual space travel (and fighting).