If you follow the same logic as twin flying, where you raise the gear once there is insufficient runway/field remaining to land on and you have a positive rate of climb, then the earlier posts said it all. Piper made retractable gear aircraft because it made them perform better, and not just in the cruise. If you can reduce drag and improve your climb performance then do so ASAP. The argument that if you are close to the ground and you lose the engine you will not have time to put it down again is a circular one, since if you had put the gear up you would have been higher and therefore would have more time. Ultimately I would suggest it's better to make a controlled belly landing into the middle of your chosen field than hit the near hedge at 70 kts with the gear down!
300 feet refers I think to flap retraction, so that the tendency to sink or reduce climb performance initially is less critical than at a lower height.
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"Take-off is optional, Landing is mandatory"