**Yes, you should charge for this since:**

1) It gives your client more control on his own content and therefore less work for you in the future or any other designer/dev... so he's saving money.

2) It will require you some time to adjust your layout to make that content editable, and it's not always easy!

3) By default on Wordpress, the posts, pages and the features of the control panel are things he can usually change. Parts of the theme are not expected to be editable unless he specifically wants this.

4) If he doesn't want to pay for this extra, you can simply offer him a "monthly maintenance contract" and make the changes he needs for him. Anyway someone will need to update that wonderful Wordpress and its plugins! That can be part of the maintenance contract.