Does it matter the wood that itomaki are made of? I’m thinking purpleheart.
Hi Shephard!
I´d say, the wood should be harder than the wood of your shamisen´s tenjin.
If you are planning to use zagane (the metal “washers” that hold the itomaki), I came to the opinion that there is a reason why ebony is used. So in this case, ebony or something closest to the hardness of ebony. Otherwise, your itomaki may get easily deformed by the metal parts and won´t hold the string in tune and will be constantly slipping (like mine do now… 
).
Another option is to get fancy and have the ends that go into the holes in ebony and the rest whatever wood you choose. But that requires a lot of preparation.
I use this site to compare janka hardness of different woods: https://www.wood-database.com/
Good luck! 