Search engines (true search engines) don't need custom feeds, they are crawler based (Google, Bing and Yahoo being the major 3) if you have the standard html / xml sitemapping module, that's all you would need, and the only one you feed xml to is Google via the webmaster tools.

If you are talking about "Trulia" "Zillow" etc - those aren't search engines, they are listing aggregators (and technically are your competition) my answer would be none of them.

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