Depends on the style, for me. For modern stuff I would think 0.3 seconds is quite enough. Most things I start from Spotify start instantly.
As for the dithering option, dithering is (or can be) important when reducing word length, in other words when you change from a higher bit depth (32 or 24 bit) to a lower one (like the 16 required for CD). Its effects are subtle but don't do it more than once, or at all if you're not reducing word length. Basically do your entire project at a high bit depth and when you make the final bounce to the lower bit depth, after all your mastering processors, dither. It sounds as if you have your stuff mastered externally. In that case I wouldn't reduce bit depth at all (nor dither) and just send the thing to the mastering engineer at your project bit depth and samplerate.