Was the /s/ in PIE retracted (/s̠/) as in modern Greek, standard European Spanish and most likely ancient Greek and Latin, or was it pronounced as in modern English?
If you mean "was there a separate phoneme *s̠ separate from *s", almost certainly not. Current reconstructions explain the data very well with only a single sibilant *s, and I haven't seen any theories that add another one (or any reason why it would be necessary).
If you mean "was PIE *s pronounced
We don't (and can't) know.
Most of the details of Proto-Indo-European phonetics are lost to time. The reconstructions you've probably seen come from comparing the details of descendant languages to figure out what common ancestor might have preceded them, which works surprisingly well for figuring out theoretical phonemes. But this method can't tell us very much about the phones, the actual sounds that were pronounced.
So when people talk about Proto-Indo-European *t, they're not necessarily making any claim about it being, say, dental versus alveolar. We simply don't have enough information to claim anything about that. All they're saying is "there's a unit whose descendants are realized in a whole bunch of
[t]-like ways, so let's call it *t".
At this point, such broad points as "was *k velar or uvular?" and "was *d an ejective?" are still hotly debated. For something more precise than that, like the difference between a retracted and an unretracted
[s], there's simply no way to know—some dialects may have pronounced it more retracted than others, or it might have been retracted more in certain time periods, and this sort of detail is simply beyond the limits of the comparative method to reconstruct. The only part we can be pretty certain of is that there was no meaningful (phonemic) distinction between retracted and unretracted sibilants.