Update.--This entry has been superseded with an update to the Green Trails maps entry and an new Web page guide to the Green Trails maps for Mt. Rainier NP and adjacent lands.
I've been using the USGS topographic maps for my work with the Mt. Rainier NP photo guide, and it's noted many of the 15 individual maps in the 7.5 minute, 1:24,000, series which covers the NP are dated, 7 from1971 and the rest dated 1989-2000. This means considerable information, such as trails, facilities, etc., are missing or inaccurate.
That doesn't necessarily the maps aren't useful. They are for a lot of uses including hiking, research, etc. That said, many hikers use Green Trails maps which are at a larger scale, 1:69,500, meaning they cover the majority of the NP in two maps with an additional 3 maps covering the eastern and southern bounday areas not in the two main maps.
If you do plan to use them, the maps you need are:
237 - Enumclaw
238 - Greenwater
269 - Mt. Rainier West
270 - Mt. Rainier East
271 - Bumping Lake
301 - Randle
302 - Packwood
And they have a special map, 270S for the Paradise area.
The first two maps, 237 and 238, aren't essential but useful for the adjacent USFS lands along the northern border of the NP. Map 271, Bumping Lake, covers the southeastern parts along the Pacific Crest. Maps 301 and 302, Randle and Packwood, respectively, have the Nisqually River boundary in the southwest and the Backbone Ridge in the southeast, respectively.
These maps are often easier to find and cheaper for the fewer number covering the NP. They provide good information on hiking trails with distances and other information, such as campgrounds, lookouts, etc. I use them in combination with the USGS maps, along with some other maps.