The fuel valve/selector could be a source of leaks, though I have yet to ever see one leak. The only part which requires a seal (O ring or packing) rotates, rather than slides. Thus they seem much less likely to leak. They certainly should be exercised. Though few 150's have them, when possible it is wise to drain the fuel valve sump under the belly. On my 150, it is the only place that debris accumulates in the fuel system. I have a sump drain there, and check it regularly.
From a design perspective, it is the possability of a leak, and fuel fed fire in the cabin, which is a major reason to not have fuel pumps in or very close to fuel tanks supplying the engine(s). You just don't want fuel under pressure in the cabin.