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Are they actually separate web sites, or topical sub-directories within the same site? For one thing, if they're actually distinct enough to be considered separate sites, you might want to go through the categories at ODP to see if you'd possibly qualify for separate listings. That is, if they're unique enough in content.
If they're in /subdirectories/ but they're linked from the main page or a site map, they'll likely be found through regular spidering, as with Google. They'll probably be found if you're already in the index. However, there really is no harm in submitting the new pages. I would, and in fact, I do. For example, on a product site, if I add a new product category, generally in its own directory, I always submit the new pages, rather than depend on them being found. In particular, I immediately submit to Lycos, since the spider will show up within a few hours for only the newly submitted pages, no others.
For changed text and/or content, I do the same. I find it's not a necessity with Google, but I always do with the others, just as a matter of routine. I even occasionally give Google an extra nudge, just in case.
You will want to stay within a limit of 5 pages per domain name (not directory) per search engine, each day.
Others may do it differently, but I haven't experienced anything negative from this practice.