What you are doing would be called tempering the springs. The temperature you cite, 500 F, would be marginal, but might trade some hardness for some toughness. The amount of time is not important, but the maximum temperature is very important. Agree that this is not necessary or desirable. Use 'em as you wind 'em. Wire diameter, spring diameter, coil spacing, and wire modulus combine into a spring constant - the amount of force required to stretch or compress the spring some set amount. If the spring is designed such that the coils completely collapse before the plastic deformation of the steel is reached, the spring will not break and will take very little set. The further you are from the plastic deformation limit, the more cycles it will last before breaking from fatigue.
