In general, picosatellites are assumed to have minimal shielding. There are two main uses for shielding the electronics and instrumentation: preventing your satellite from being noisy and protecting your satellite from space.
The first is necessary: shielding your electronics so they do not put out radio frequency (RF) noise that contaminates your instrument measurements, your own radio transmitter, or other satellites nearby. Given the low power levels involved, the last bit (contaminating past your own satellite) is unlikely and such noise would be a result of very poor parts choice or assembly, rather than being a shielding issue. Shielding for internal noise may require specific grounding or small bits of foil covers, but neither introduces a serious weight factor.
Shielding your electronics from space, whether it be from the varying ionospheric interactions in orbit or from catastrophically large solar (space weather) events, is more problematic. The first problem is that shielding requires weight. The second is that shielding is subtle. Incident particles (charged electrons and protons) can do transient and long-term damage to your electronics. However, transients can be recovered from, and a typical picosatellite will not be in orbit long enough to accumulate long-term damage.
More critical is that shielding is not a simple matter of adding material to make you safe. Damage occurs when charged particles or EM fields penetrate the surface layers of your satellite and ...