> Testing start bit would be certainly helpful to avoid false
> reception initiated by spikes on the RB0 pin,

Hardware UARTs/USARTs resample the input signal 1/2 bit time
after detecting an edge.  If the signal is still there, it is
considered a valid start bit.  If not, it's ignored as noise.

> but testing stop bits seems less useful to me. Even if you
> discover framing error what would you do ? Trash the byte ?
> Ask for resend ?

Hardware USARTs generally return a framing or overrun error
and (I think) along with the data which was received.  Upper
level software then chooses what do to with the octet (ask
for resend, etc).
                                                Lee Jones