noo! It is quite uncommon, be careful of a bad mistake in use in Rome:
'essi' in place of 'sii'; it is wrong but more common then the correct 'sii'.
Sometimes a mother could say to a little son 'sii buono, smetti di ...';
a sport coach could use the plural as in 'siate pronti', but the singular form
is usually 'stai pronto' (verb: stare)