Regardless of whether you supply an assembly layer & centroid file as part of your gerber/cad data, having no Silkscreen whatsoever is generally speaking, a bad idea. At the very least the silkscreen should contain useful markers for device orientation, connector names, voltages & test points, maybe a nice spot for a serial number and firmware to be marked with a pen and generally enough to make the board "navigable" without reference to drawing or documentation. Don't forget that once in the field, someone may encounter your board who doesn't have everything to hand but does need to interact with/repair it.
In theory if every CAD package and SM machine defined their components according to IPC standards you could send a centroid file and that would be that. However in practice people do not define their component orientations according to those rules which means a contractor needs to verify orientations visually against either the assembly layer or the silkscreen. If you define all the key items on the silkscreen you can also carry out a secondary check as part of your NPI process using the fiducial camera to step through each component location and overlay the device outline&orientation over the PCB image. For low volumes where some parts might end up being manually placed a silkscreen is helpful to find those locations, and if your board is weirdly symmetrical it might even help with getting it the right way round.
IME using Essemtecs centroid import tool most people define 2 pin non polarised devices the same way. But notably two recent clients do not, this doesn't seem to be a CAD package thing either as one uses Altium (as do we) and the other Cadstar which hasn't in files I have previously encountered. Presumably both of them have defined their own libraries from scratch and for whatever reason defined these 90 degrees out. By contrast ICs etc almost always require a 270 rotation in the import utility, which suggests that here it is Essemtecs templates that are rotated (and there is little point checking seeing as there is not much I can do about that). Thats why the Essemtec software (and one assumes everyone elses that is remotely focused on NPI & small batch) builds up rotation rules for components and packages which are stored, in my case, on a per client basis.
It is entirely possible (quite why you would is beyond me) to spec some SM machines with no CAD import facility meaning they cannot import data from a centroid file instead relying on either manually teaching the entire board using the overhead camera, which is basically impossible without silkscreen as well as being painfully slow and prone to error or by using a 3rd party tool like PCB synergy to go directly from CAD data to machine specific format. I have encountered contractors for work we considered too much for our process who set the board up using manual teach, I have no clear reason for them doing it this way but it seemed to basically boil down to the operator having no clue how to handle data that wasn't in exactly the format his Mirae line would take as input, given he had the file as both XY and full CAD data there is no excuse for it. PCB Synergy and Libreoffice are both free and both could probably have carried out whatever "conversion" he thought was needed.
It is also possible (as mentioned previously) to create a centroid file from the gerbers using GC-Place or FAB3000 or Unisoft, again with these tools you need either a silkscreen or assembly layer as part of your output, again these tools take time and introduce the opportunity for human error (typo or misread) so I consider these tools "last resort" weapons when the client cannot or will not provide a centroid file.