You should only re-ping a site,page or blog if you update or make significant changes to it. There is no need to continually ping the same page or site and to do so could be frowned upon.
You can however ping each of your new hubs individually whenever you publish them and again only if you update or overhaul them in the future. Big established sites like hubpages get crawled regularly so it's not too big a deal here.If using different ping sites you also need to be careful as some of them may ping the same search engines/bots etc.and you could be duplicating (or more) your efforts.
Pinging is not a necessity but it can draw attention to your new or updated sites/pages somewhat quicker than would otherwise occur.