I'm trying to extract harmine from seeds of Peganum harmala. I read a step like: saturate with salt to precipitate harmalas HCl. Is it necessary for the solution (on which salt is to be added) to be acidic or neutral? How about if it's basic? Ideal pH?

It has to be acidic but you don't need to know the exact pH cause if the pH is too high (basic) harmalas are in freebase form which are insoluble in water so there is precipitation. In this case add more acid (vinegar, HCl, ...) until there is no more precipitate and then you can put salt in the solution.

I guess at basic pH some unwanted alkaloids precipitate too so that's why acidic pH is needed to have them dissolved.

Aum_Shanti had here shared this: (the below quoting Aum_Shanti by me not necessary be exact copy)

So seeming to me that too acidic pH is not desirable.

Ideal pH does not exist, you're really over thinking it. If you want a number: 3 to 4.
Whatever not dissolves at 3 to 4 is not what you want and those solids should be discarded. In the first 1 to 2 manskes that could be quite a lot. Filtering can be hard, so letting the undesirables settle at the bottom works better.

Keep repeating manskes until pH 3 to 4 does not produce solids anymore to discard. Then have your final manske if you prefer the alkaloids in their salt needle form as most do.

Even at pH 5 all of your desirables are dissolved, so 4 gives you headroom which might be needed when using pH paper that is not so exact by times.

I think the title question is a bit misleading.
The big question is, what do you understand by a "Manske"?
The problem is, as Jees already stated, to first convert the FB into a salt form. For this you need the acid.
But e.g. if you happen to have already a salt form, you can just dissolve it in pure water and add the NaCl...

That's at least how I understood the process. Please correct me, if I'm wrong.