If the battery pack was flat for only a few hours, at most a few days, then the above advice is fine. However, if the packs are kept flat for weeks or months, then toss them in the bin. When over-discharged(flat), re-chargeable lithium cells form microscopic conductive "tendrils" or "fingers" that breaches the electrolytic barrier between the electrodes. This results in higher self-discharge and reduced cell capacity - and in the worst case scenario causes internal shorting and possible fire/explosion - typically under charge and with no warning.
If you accidentily flatten a lipo, then perform recovery immediately.