I think the answer might have to be:  "it depends" -- on how you develop the starbase, and on the strategic situation.

A starbase that is heavy on strikecraft and light on direct weaponry / abilities should be far from the attackers, for instance.  An Advent starbase with max'd confusion / meteor-smackdown abilities, both of which appear to affect all targets within a given radius, and with the armor upgrades to survive, might want to be in the thick of the fight.

Setting up very close to a phase line, surrounded by mines, repair stations, and beam platforms would mean you'd be immediately engaging arrivals.  They'd have little time to quietly clear the mines or to attempt to go around you for easier prety, or to pick off your structures from long range.  On the other hand, if the attackers is heavy on short-range firepower and abilities instead of carriers and long-range anti-structure ships...

In addition, if there are multiple ways in (and against a Vasari player with a Kostura in the same system, there *is* another way in), well... unless it's a star, you can only place on starbase.  And even starbase weaponry has limited range.

Addition:

Oh, and regarding the phase line disruption:  It doesn't matter.  Anywhere in the gravity well is fine, as far as I can tell.

Good advice on the starbases leejaywu. 

Personally I play the TEC and have two strategies for placing starbases.  My overall game stategy is to fortify and upgrade every planet completely, as well as building the max number of structures possible.  I also put pretty much all my structures in a small area so the tactical structures can defend the logistic structures.

On planets that have a low tactical count (25) I build three or for hangar platforms and fill the rest with gauss platforms (when all the upgrades for these two structures are researched, they are very powerful for the size), and put the starbase close enough to support the gauss platforms and hangars.

When there is a high tactical count (35) I keep the same tactical stuctures (but more of them, obviously) and put the starbase on the other side of the system. to split up the targets and have fighter attacks coming from different angles.

As for starbase upgrades I always totally upgrade the weapons systems and hull points (I fell if I spend the time and money on a starbase, it should be able to last through a hard fight), and give at least two upgrades to strick craft (giving me eight squardrons in addition to whatever I have from my hangars).  The last upgrade point, I give to the orbital government if it's by a planet, the trade port upgrade or the last strike craft upgrade if the starbase is defending say, an steroid field with just a couple of metal extractors.

As a final note, I LOVE starbases.  Yup, LOVE them.    On the easier difficulty levels the computer won't even attack a system with a fully upgraded starbase and a full compliment of defense structures.  That leaves you free to go forth and CONQUER!