Definitely bind the book first, and THEN write in it - this way you avoid the (often unavoidable) OH-SHIT-I-JUST-SEWED-HALF-MY-BOOK-IN-UPSIDEDOWN-AND-ONLY-NOTICED-AFTER-I'VE-GLUED-IT-THAT-WAY problem
Without writing, when you make this mistake it's called 'rustic and handmade'. With writing, it's called being an incredible dipshit. And I can say this because I've done it many a time.
Also, if you write first and bind later, you may find some of your writing gets lots in the spine if you bind it too tight, so you risk not being able to read the first word of every line.
Hope that helps
Happy binding!