Is attempting to get published in undergraduate history journals worth it?
First of all, if you've already written an excellent paper of which you are proud of, then why not submit it to an undergraduate history publication? It certainly won't hurt your chances for graduate school admissions. In fact, I would say that it certainly demonstrates interest and willingness to engage in the academic community of your chosen field. This is something that graduate admissions look for and care about when they're evaluating your application.

Undergraduate publishing opportunities are quite plentiful, and many of them are becoming very well-recognized. In my opinion, I've found that the writing/research published in the best history journals are neck-to-neck with papers that could pass a graduate thesis defense.

I would encourage you to check out:
http://history.unc.edu/undergraduate-program/undergraduate-journals/
with an emphasis on Yale's and Vanderbilt's undergraduate (which are one of the few journals open to undergrads from all universities rather than just serving their own university/college). (Full disclosure, I am an editor of the latter, see: http://vanderbilthistoricalreview.com)