Nietschze criticised Christianity for espousing slave morality. How true is this?
The fact that Nietzsche proposed such an analysis is very true. Whether or not you subscribe to Nietzsche's analysis is another matter; in any event, you'll have to read it carefully before choosing to accept or reject it.
Given that Christianity became the religion of the ruling classes, isn't it more correct to say that slave morality is espoused by the ruling classes, not for themselves, but for the ruled?
Nietzsche is critiquing the morality of the ruling classes of his time, who are, indeed, Christian. It's not therefore "more correct" or "less correct" to say this; it is a fundamental part of Nietzsche's argument.
After all, if Nietschze was correct, then in todays post-christian & secular western europe slave-morality should have withered away
Not in the slightest. Nietzsche calling Christianity a "slave morality" does not mean it is the only form of "slave morality", and there is absolutely no reason he would have us believe that today's (allegedly) "post-Christian and secular Western Europe" would be any less a slave morality; the fact that it is committed to the notion of "democracy" should be indication enough (as, for Nietzsche, the democratic impulse is slave morality par excellence.)
but though it is no longer culturally espoused, the rebel being an iconic figure, as opposed to the saint, it seems to me practically this is still the real morality, for example, espoused by governments & corporations.
Again, you seem to think you are arguing with Nietzsche when you are merely recapitulating a small part of his argument (although I think he'd say that you are being capricious if you think that Christian morality is no longer culturally espoused...)
According to Wikipedias entry on master-slave morality, master-morality is the morality of the strong-willed, and slave-morality is the response of the slaves.
That's a bit reductionist (as one would expect of Wikipedia), but not too far off the mark.
Whereas I'm asking isn't slave-morality foisted on the slaves by the masters to keep them in a state of submissiveness.
No, the other way around. Slave morality, Nietzsche argues, was foisted on the masters by the slaves, in order to keep them in state of submissiveness, and that's what Nietzsche finds so wrong; the strong-willed are absent, or in hiding, and need to awaken and seize back what should be theirs. (And that's more than a bit reductionist, as one would expect of a brief answer here.)
To summarize: if you're interested in Nietzsche on slave morality, read some Nietzsche, or some of the (voluminous) secondary literature on Nietzsche; Wikipedia is barely a start.