Aside from sampled instruments, there's the entire world of synthesis, including the endless number of virtual instruments that have become staples in modern popular and electronic music. As to your question: they are not generally stretched tuned. Many instruments support alternate temperaments through the use of .tun files, but temperament (with a few outlying exceptions) only speaks to the interval deltas of the notes within an octave, not of the entire range of the instrument.

However, it doesn't matter. The reason for the stretch-tuning of a piano is due to the physical limitations of a vibrating string being unable to produce the same harmonic intervals at the extremities of the instrument's range, and does not apply to electronic instruments which, particularly in additive synthesis, can generate whatever harmonics are desired, including harmonic configurations that no physical instrument would be able to produce.

A lot of keyboards are based on recorded samples of acoustic pianos – they might use a Steinway or something as the source of the samples, so the keyboard and piano would sound ostensibly similar and good together.