With the new development strategy there will be no version called LTS upfront. With the information currently published 3.4 will take the place of the announced 3.5. Assuming there are plans for 4.0,4.1,4.2,4.3,4.4 at what time sites are expected to be migrated to which version.
There's a few things to consider when planning migrations between major versions. One of the top ones should be the end of support or EOL date for the version you're using now; if that is close, you should be planning your move.
For established websites, I'd suggest looking at the available features. Does the new version have something you need soon? If so, start planning your migration. If your site is working just fine and doesn't need the newest features, you can stay on the current version as long as it's still supported.
If you're migrating, what's the extension support look like? Do the extensions you use have a compatible version for the version you're migrating to? If so, that simplifies things greatly, but if they don't, how much effort will go into replacing their features with something else?
Finally, every release that's tagged as stable should be suitable for building on, but personally I'd be a little more cautious about using a X.0 or even X.1 release as those are typically more prone to having bugs given the scope of change that can go into a major X.0 release. I personally think X.2 or X.3 releases are when you start to see a release series mature to the point of being ready for major use.
Starting from Joomla! 3.4 every version will be a stable one and will have support at least two years unless the new minor version will be released. If it will happen then the previous minor version will not be supported and the current one will be supported at least two years. So if you are planning to migrate to 3.x series, then you are safe do it from v3.4.
The same applies to 4.x series - 4.0 also will be a stable one and will have support at least two years unless the new minor version will be released (for example 4.1). So you can migrate to any minor versions of 4.x series. But keep in mind that every major release has backward incompatible changes! So this migration should be planned very carefully.
Of course you should not migrate to BETA or RC releases, because these are for testing purposes only.
In my understanding, starting with 3.4 you should always use the latest release, as it will get support for 2 years after its release and support for older versions (3.3, ...) will be discontinued. All releases in a series (3.x) will be backwards compatible so an update should generally be no problem.
The new 4.x series will break backwards compatibility and you should only upgrade then if you want to use the new features.
- If you starting development, then use latest major version available (Use 4 series when its available)
- When a new minor version (3.4) is released, wait for the time your current version(3.3) is supported. In the time the new version must have received critical bug fixes. So your website can now be updated safely.
Tip: Always maintain a test site with same server setup, to test it first, then use the version.