For SD, I hope that, if they are giving information, it is at least correct. (I have already seen epg data on other providers continuing to announce a specific show at its usual timeslot, though it was not broadcasted anymore).
Concerning other providers: I don't know for tv_grab_it, but webgrab+ offers several providers for most channels. The user has to create a configuration file that tells webgrab+ what channels to download; it is possible to freely mix providers in the configuration file.
But for the moment, I am trying to find an epg source with accurate data and working over a long period without having to continuously do adjustments.