I guess a lot of people are interested to know if pinging for individual blog url like http://www.myblog.com/2007/1/10/i-just-blog-today or pinging for a category feed like http://www.myblog.com/rss.php?feed=newly-added-category will end you up in the spam list. Basically what this means is when you had just updated a post, should you ping for this url that is generated for this post? Similarly, if this new post is filed under a category and there is a unique rss feed generated for this category, should you ping for this rss feed too?
Well, I hope this is not too confusing. In my opinion, as a ping is delivered, we basically pass in 2 parameters like blog url and blog feed. So, by pinging for the unique url generated after each post is fine because we are letting search engines and blog aggregators (endpoints) know that we have a new post at this particular url. On the other hand, if you are trying to ping for a different feed, then this is not advisable. The reason being that you are still pinging with the same url but a different feed and this may cause the endpoints to suggest that you are spamming with your blog url.
Conclusion, never ping too often with the same url and you will stay clear of the spam list. Hope this helps :)