Continuity is such a subjective thing with Kamen Rider though; you could actually make the case that BLACK must take place in a seperate timeline from everything else since
[HIDE]Japan is in really bad shape at the end. As in "Maybe we should move away, like, forever" bad shape, with roving monsters, mass mayhem and questionably acted gang members running amok. And I don't care how much time elapsed between that and RX; realistically, things would take years to recover from that. So good thing we're not talking about reality![/HIDE]
But of course we know that the other Riders appear in RX (spoiler warning in case you missed the 1980's or something), so obviously they had to be around during the first series, traveling the world or something. Or in Arizona, punching cacti.
So going by that logic, I can totally buy that the Heisei series share a continuity.
Here's the thing though: officially, it's just never specified. It is never explicity said "Yes, they do take place in one continuity" or "No they don't." However, the attitude *most* of the Heisei series take is that they are a self-contained story, seperate from any other series. They don't outright say it, but they don't often refer to other series, and if they do it's either a very loose or superficial link at best.
(Consequently, the Shôwa series are also self-contained but in a less restrictive way. But even then only a few of them actually rely on events from a different series; a lot of the time the past Riders just show up to help out because nobody said they couldn't. There's a reoccuring villain yes but I look at that as just a joining factor that's more metaphorical than essential.)
Putting aside the upcoming Kiva/Den-O movie for a moment, Kuuga & Agito are the exception in the Heisei series, but even there the links are minor and you could watch Agito without ever knowing about Kuuga. However, it is softly established that Agito is set in the same world as Kuuga, which in turn may be set in the same universe as the older series (Getting into spin-off material is messy, but the Genealogy of Justice PS2 game sort of supports this theory; there's even a timeline in one of the guidebooks. It's an interesting idea at least.) They don't really push the shared continuity, but they still keep it open so you can make your own connections. I like this because while I think it makes sense that V3 doesn't just wander into Agito's show (since it wasn't required to have him), it still leaves open the possibility that some day they could feasibly meet.
It gets a bit trickier from Ryuki onwards in part because that's where the alternate timeline movie/ending craze began. Before Ryuki, the movies didn't always fit in perfectly in their series, but you can BS a fairly good excuse for all of them (well, I can BS a fairly good excuse for all of them at least.)
Post-Ryuki, I think if you're familiar enough with the endings/the movies of all those series, you can see how they could/couldn't connect with each other and with previous series. As for the content/plots of the series themselves, there's stuff that conflicts, there's stuff that doesn't, but I think with all the series there's a vagueness to them where you could make up a trillion theories but none of it will ever be confirmed or denied by the series because let's face it; when a Rider series ends now, it ends and that's it. Unless it's Den-O.
On the subject of Den-O/Kiva... I'm just going to wait until I see the movie. I have a theory though that they'll basically just take advantage of that vagueness factor and set them in the same world and then never talk about it on the Kiva TV series because it doesn't apply to the plot there, but... we'll see.
I personally believe it's possible for all the Rider shows to take place in the same universe if you're willing to bend the rules and pick & choose what movies/endings "count" and which ones are timeline spin-offs, but Toei has always been so vague about it that there's really no way to ever come to a definitive conclusion that isn't just your own theory/speculation. I have mine, you all have yours, and that's about as far as it'll ever get.
One more thing: I know someone will bring up Hongô (from THE FIRST) appearing in Kabuto as part of this grand connection foretelling a crossover, but it honestly, it just turned out to literally be a throwaway gag for fans. Or a half-baked "Homage to the past since it's an anniversary year", take your pick. So I don't think it was ever intended to mean any more than that. In fact, people on some Japanese boards are now speculating if he'll have another nonsensical cameo in Kiva!
I'm half asleep so hopefully that rambling made some sense.