the most rez you'll ever need is for high end litho printing, ie a 24x36 (inches) poster, after that, rez starts to go down.
now there are a bunch of other usages, mostly large format, that require X size @ 150 or 200 DPI, but a billboard is aorund 50-75 DPI @ final size
SO, communication w/ client is essential to make usre all the bases are covered. a competent studio person @ the agency should be able to answer your questions
the digital to film and then scan is a solution I've seen work in the past as well...
jim