"+tossgren" primes a grenade (and throws it when you release the button)
"tossgren" throws a grenade (timed or impact, depending on the server setting)
"tossgren timed" throws a timed grenade on any server
"tossgren impact" throws an impact grenade, but only on servers with gren_explodeonimpact 1 set.
Really all you need bound is +tossgren and tossgren. If you want to throw a timed grenade, you can just quickly press and release +tossgren. If the server has impact grenades inabled, you can just use tossgren, and it's an impact grenade by default.