At the time the PCs are able to do planar travel by themselves they can fully start to explore potentially hostile planes. Since it's only starting at level 13, the power to do so is extremely uncommon, and very few things are known of the other planes except general cosmogonic stuff... And If find that the campaign is over before I feel the need to introduce other material planes. The prime material plane of the PC is enough for many adventure, add a handful of plane to visit and you're at 20 quickly... (if the campaign doesn't peter out before like, apparently, most do). So basically, no, I have never used the multiverse, I have never felt the use for the multiverse and I feel few playing groups reach high level in order to make that a feasible activity.
On the other hand, the last campaigns our group had, all set in Eberron, had nods to each other. A PC bard from campaign one is a large pop star in campaign three, the main exhibit hall of Morgrave university is named in campaign five after a group that made a large donation in campaign two and so on. Informally, the players accepted that a million of alternates Eberrons popped in 998. My next campaign will probably let them investigate the Great World Fracture of 998 as the tier 4 act of the campaign, but I haven't really no idea how it will play out (since I don't want effective reality hoping, but probably involve something to do with Xoriat...