Most blog providers support pubsubhubbub(the worst protocol name ever). It allows you to avoid polling, by having the content producer notify you of feed updates.
Except that, even with PSHB, you get polling. I wrote about it here http://www.dp.cx/blog/pubsubhubbub-and-polling.html#.UUcPWLp...
The fact that two feed clients, Superfeedr and Guzzle, implement PuSH wrong in no way leads to the conclusion that, when creating a feed reader, you can avoid polling for every content source that does support PuSH.
If you have at least fifty previous entries from a feed it's easy to predict fairly accurately when that feed will need to be updated again.
That's how the indexer for http://rssident.com works. Saves you tons of cpu cycles.