One of the most frequented PSK31 frequencies is 14.070 kHz. However, you will also find PSK31 activity on the following HF frequencies.
Please note that the frequency displayed on your transceiver is the suppressed carrier frequency!
BPSK - binary phase-shift keying - is the most popular mode of PSK31.
Which Sideband To Use?
It will not matter which sideband you use to work BPSK31 signals. You can work any BPSK signal that the PSK31 software displays in the "waterfall" window.
Do keep in mind that your transceiver sideband filter will pass all signals within 2 to 2.7 (even up to 3 kHz in some cases) above (if in USB) or below (if in LSB) the displayed frequency on your transceiver.
Note that some 90 PSK31 signals can squeeze in 3 kHz, side-by-side! If a signal within the passband is very strong, your transceiver AGC will reduce the RF gain ... effectively wiping out the weak PSK31 signals within the passband.
Interfering Strong Signals?
There are two solutions to this problem.
Install and switch-in a narrow IF filter if your transceiver is designed to accept one or more.