I'm sure others could speak to this more, but personally, I only reached out to profs when I genuinely had pressing questions. And for me, that amounted to only a couple of times. If you're nervous about whether it is appropriate to reach out, you could consult the DGS or the point-person for admissions for the program in question and ask if professors are generally open to prospective student inquiries. If they are, then you probably don't need to worry too much about what exactly to say! Some programs, the more competitive and "highly ranked" ones, actually encourage people not to reach out unless it's about a procedural matter. Most seem to be fine with it, though.
Perhaps others have had different experiences, but for me, it seems to not be too consequential whether you do or don't communicate with professors. It probably won't hurt, to be sure, but when it comes to admissions decisions it won't likely make much of a difference. So, again, if you have pressing questions about a program or an extraordinarily keen interest in particular profs, I say go for it, send an email and just be polite and be yourself!
For context, I'm a current MA student and I'm applying to PhD programs for the first time--so obviously you should defer to more experienced opinions, if anyone else replies. But I will say that judging from my peers' experiences, corresponding with profs didn't seem to affect anything either way, or at least not in any way they could tell. I have friends who are now in top-10 PhD programs who contacted profs in programs they never even heard back from, but did not contact any profs in the programs from which they ended up receiving offers. I'm very curious to hear from others on this, though.