You may find it more useful to use the CDU tuning by clicking NAV, COM, or ATC on the left side of the CDU keypad. No decimals required, and even if you don't have the RTU's in 3-digit mode it works great.
Neat thing about the NAV tuning is that you can enter the 3-letter identifier and it'll auto-tune the frequency, clutch in a "slam dunk" situation when you're scrambling single-pilot to prep when ATC is running at a high tempo.