I think you'll find for the Imps it's simple maths - especially with the linear progression of rewards this time out. Most people don't have time or inclination to compete for high levels of frags on EVERY world - especially given the reduced time scale these events have.
So what makes sense, time wise? Assuming no perks involved, it's a minimum of four attacks before you can planet-hop...
Six planets of 5 frags = 30 frags.
Four planets of 10 = 40 frags.
Three planets of 15 = 45 frags.
Two Planets of 20 = 40 frags.
It's fairly obvious which planets most Imps would go for first... Or, for that matter, decide to skip.
(P.S., in the desert conflict, I managed that rare beast... the 100% attack on Tatooine. And promptly (and suspiciously, frankly) crashed at the end and lost almost all the progress I'd made up till then. That was enough to put me off even trying to continue for 5 frags...)