hey there modarazzi
i would lean more towards the "asap".. the python script that i use to build my sitemaps, has a (suppress_search_engine_notify) option to disable/enable SE notification upon sitemap update.
although you can give the frequency and priority of individual urls, i think it's similar to the meta tag setting saying how many days for the robots to revisit your site.. the SE's probably take that in to consideration, do some backend processing to determine the relevancy of how often to really scrap the sites.
calling a SE on command would be really cool
i have been fortunate that SE's come to my site quite often due to my frequent content changes.