Given the logistics/cost issue with starbases, once your "front line" with other civs has advanced, is it better to decommision some star bases further back (along the old front line), especially influence ones that are no longer needed??
Absolutely decomission influence star bases that aren't around enemy planets unless they are in danger of being invaded. There is nothing more satisfying than an enemy invading your planet (which sucks) and having it flip back to your control a couple of turns later. But this is not very easy to gage, so yes. Destroy them.
The Military star bases are a more complex question. Certainly, if it means not being able to make a new star base in the New Frontier, I would say yes. But otherwise, i tend to keep them. This creates a second line of defence against invaders. You never know when the tides might turn. Also, the mere fact that they are star bases, make them High priority targets for the enemy. So they will waste time and effort killing your star bases instead of attacking your planets. That will allow you to shore up your other defenses (at least IMHO).
And if you have a fleet chasing you (through several layers of "Frontier" defense), the warp disruptors will allow the remnants of your damaged first wave to regroup with the fresh second wave and crush the enemy.