Steve Harris uses flats now, but everything up to Seventh Son (all the classic stuff) at least was roundwounds. His recorded sound is woeful these days in comparison , and i don't know why anyone would want to copy his current gear to emulate it. He also restrings every gig, which is very unusual for a flat player.
Geezer Butler uses DR Strings (Hi Beams and Black Beauties I believe).
Nothing against flats, McCartney on Abbey Road and Pino on D'Angelo's Voodoo are 2 tones I could listen to all day and it's great that they have made such a strong comeback over the last few years, wouldn't have thought it possible 10 years ago. I just think that it is important to put forward another perspective.
Nearly everything recorded from the early 70's until a few years ago was roundwounds. Sure there were still some examples of flats usually in bands trying to recreate the vibe of the sixties, but they were certainly not common. One of the major reasons for James Jamerson's decline was his refusal to swap to rounds. He found himself unemployable because his sound wasn't as suited to the style of the day. What was great in the sixties wasn't up to par in the seventies.
I also think it's a bit misrepresentative to say that rounds are ok if you want to sound like John Entwistle or Chris Squire. Both great players with really distinctive personal voices that would difficult to emulate with any gear. What about choosing players like Tony Levin, John Paul Jones, Jaco Pastorius, Victor Wooten, Leland Sklar, Marcus Miller, Rocco Prestia, Geddy Lee, Stu Zender, Doug Wimbish, Flea, Nathan East, Steve Digiorgio, Bernard Edwards, Robert Trujillo, Rex Brown, Jack Bruce, Anthony Jackson, Eddie Jackson and Mike Dirnt. That will give a more honest perspective of the true sound of rounds.
As said, I have nothing at all against flats. I have a bass strung with a set and love playing it from time to time for variety. I'm sure that you can make them work in any style of music, but if I was doing anything other than a sixties revival or a soul band, flats would not be my first choice.