Truth to tell, I've never really given it much thought. I like hickory or shiro kashi for bokken as they tend to last the longest, and a good sized tsuba to protect my hands from over-zealous partners is nice, but that's about as far as I've gone when thinking about bokken. In the arts that I've practiced, they are considered consumables and will get beaten up from contact. Therefore, not a whole lot of thought has ever been aimed at things such as balance or weight.