Just keep in mind that a sharp cut creates a phase shift around the cutting frequency, so even though you can't hear much below 20Hz, the phase shift created by the EQ will definitely affect the overall balance in the low frequency range, so you need to be careful. Using a linear phase EQ might help, but you need to watch out for transient smearing. To summarize, I try to avoid sharp cuts or boosts on the master, and always get rid of unwanted frequencies on individual channels or busses.
Article by Ethan Winer regarding EQ and phase shifts and what your sound really experiences. Interesting read nonetheless
http://ethanwiner.com/EQPhase.html
Articles are great, but it's always useful to check for yourself. Open up any synth, make a simple saw wave patch, and play a low note (say, C2). Then add an EQ with a sharp cut around 30Hz. The fundamental of the C2 note is around 65Hz, so you should hear no difference, right? Try for yourself, the difference will be quite audible. It will become obvious if you look at the waveforms (1st one is no EQ, 2nd is with EQ - I used FF Pro-Q2 with a 36db cut at around 30Hz):
Moreover, without the EQ, I have -6.8db of headroom (top picture). With the EQ, all that headroom disappears due to the phase shift introduced by the EQ. This can be fixed by switching to linear EQ mode (in which case the waveform will not be affected), but linear phase EQ adds smearing to fast transients.
