.38 Special +P runs up a bit more pressure than the standard .38 Special (18.5k psi vs 17k) but achieves higher velocity by using lighter bullets. So it basically is the old "slow and heavy" vs "fast and light" argument that will never be settled unless or until the "progressives" confiscate all our guns and herd us off to the death camps.
With the relatively small difference in pressure, there are likely not many guns that would be OK with standard ammo, but would damaged by firing +P. Still, with older guns, I will weigh in on the side of caution.
(+P+, though, is a whole different ball game; some boxes advise use only in revolvers chambered for the .357 Magnum.)