Hi Rebecca,
I've been collecting GIS data and maps for over 20 years, so things have changed considerably during that time. Presently, this is how I operate:
GIS data --
Focus on acquiring local data from counties (and maybe cities) that your patrons are mostly interested in. By doing this systematically, you can develop a data archive that will be useful to future researchers who want to study the past. If some of the cities and counties already have an online archive of data, then you may not want to duplicate it however. If some cities and counties require data sharing agreements and purchases, then it will be very valuable to your patrons if you can work those out so that you can have the data on hand for patrons to use so they don't have to go through that themselves. I did quite a few sharing agreements years ago, but very few local governments bother with them now around here, but I don't know about where you are.
Also remember that local data is updated daily, so you just have to settle on an approximate time period to take "snapshots" -- annual snapshots are usually adequate. The home county we are located in provides monthly snapshots on their website, so I send people there instead of our own collection.
Depending on how well your state government is archiving data, you may or may not want to start a State data collection. If their data is easily accessible online already and is being archived, I would not duplicate it. On the other hand, if there are barriers to access, then the library can help reduce those barriers for your patrons.
I would not collect any US Federal data since most is already online, unless there's something special about it. International data is very challenging, and usually must be purchased, so we only obtain such upon request.
Honestly IMO, major improvements in the past ten years or so in data streaming services (especially for imagery), open data portals, cloud storage access, and sites such as ArcGIS Online and Data.gov have really reduced the need for investing in building a large and comprehensive on-site data collection -- unless there is significantly valuable content that is not otherwise being archived.
Maps --
We are no longer acquiring any new maps unless it's a very special circumstance. We have weeded down to just North Carolina topographic maps, and a couple of cabinets of older depository and other assorted maps. All are in off-site storage and there have been only a tiny number of requests for them in many years.
Jeff Essic
Lead Librarian for GIS
North Carolina State University
Hill Library;
Data & Visualization ServicesBox 7111, Raleigh, NC 27695-7111
(919) 515-5698
http://www.lib.ncsu.edu/gis