One on each planet is redundant, since culture spreads several jumps before it starts crawling so slow that it'd take ages to get to the next. A single hub can usually go for 3 jumps (though it also depends on how many phase lanes are leading out from the planet since it is split). Culture has to be continuously generated to provide the benefits, so if you scuttle all the non-frontline hubs, the ones left may not be enough to reach your backwater planets and you will lose the allegiance bonus.

So you don't want one on each planet, but you do want your whole empire to be covered. There's no magic formula for how many you need and where, just play it by ear to make sure your whole empire is covered.

They are a bit more agile, it should affect turn speeds and possibly acceleration/deceleration. Speed should not be in an equation that uses mass, and even acceleration is a toss-up.

Thanks for all the quick replies guys ! 

it does help if you want to defend your internal worlds against a vasari kostura cannon attack (its secondary ability creates a temporary phase node at the target planet, so your ships can jump straight there from any phase gate in the same galaxy, in case you didnt know)

also, having a good coating of culture is a semi preventative defense against a deliverance engine attack. sure, one burst wont 'destroy' the planet, if you have higher than normal alligence, the superweapon is less effective.

but, there are other bonuses, such as higher tax and resource income. that reason alone is often enough reason enough to keep the culture flowing

personally, i place them either on my front line worlds and/or on worlds one jump away, as well as planets with lots of phase lanes coming out of them. sometimes, if a planet is at the end of a fairly long dead-end 'run' or 'line', then ill probably put on there also, just to help it spread.

basically, you should keep it spread everywhere, then you can scuttle as needed. but if culture starts to recede, then rebuild that culture center.

also, im sure you know, the more broadcast centers you have, the faster and more inexorable the culture spreads. so, if you want to use it offensively, to support your fleet, build 3-5 centers on a border world, and hope he has fewer/less effecient centers than you then once you expand you can scuttle.