One of those things noone will ever know without inside knowledge, you can make educated assumptions on either side of the argument but you never really will know.
As for me, recently had 12 accs spread across 3 IP's (VPS+Proxy, 4 each) All got sweeped the same night. What I take from that is that the rest of the accounts operating on that address will get searched, but as for the IP being flagged and constantly monitored, I doubt it personally. Otherwise I'd be getting bans out the arse for using the same proxy on 4 accs for a month straight.
Again, very subjective, depends on your experiences. I'd say I've experienced both but I'll never know if that's the case.