ATC usually provide the recommended spacing for VFR approaches but mandate the time for departures - haven't ever heard of allowing the pilot to make his own decision on a departure.
You could ask tower if it is an option to take that separation yourself, I don't believe they would ask you first on that one, since you may inadvertently end up putting a pilot in harms way (if they misunderstand the intention)? I don't know about the local rules for wake turbulence separation for VFR where you fly though?