It's particularly important to hide the sidechains of hydrophobic segments in the core. Hydrophobics become exposeds when they're too near the surface. Every protein in Foldit is assumed to be surrounded an "aqueous solution" of water and other chemicals. The water is never visible, however.
There's a "hiding" score part for every segment. The hiding subscore of every segment is negative, including hydrophilics like lysine and arginine. Hiding the larger hydrophilics in the core is generally not possible without creating lots of other problems. Smaller hydrophilics like serine may be found in or near the core at times.