That depends, if the site's index count is low which means that only a few pages are listed, then submissions by PositionCare may be the perfect answer. If the site's index count is good and rankings are low then the site's content needs to be enriched. PositionCare's Tutorial pages provide a good starting point for optimizing the site. Submitting a web site weekly or monthly serves no purpose and can adversely affect the web site. You can choose which pages of your web site you want to be submitted to search engines on the Manage URLs page.
Web pages are either static or dynamic.
It will not change unless somebody alters the content and uploads it to the server again. Dynamic pages on the other hand are created on the fly and contain content which is generally populated from a database. Static pages are almost always submittable to the search engines however dynamic pages can pose a problem. Spamming comes in several forms. Over submitting a web site or submitting to many pages over a given time frame would both be considered forms of spam by many of the search engines.