If you commit to the buy and it drops you should be managing your bankroll and buying as it drops all the while if something doesn't look right check the news on the coin through reddit and twitter and depending on if there bad news, keep your trade available for a pull out in the 10-20% loss region.
If you feel its just a late rebound or if the previous chart history shows similar drops and pullbacks then you could keep it in for the long haul and stay invested in that coin until it comes back for breakeven or 10% profit.
this would be my idea of trading in this situation which is a downfall and probably takes a good bit of experience reading charts to understand when you are in this situation. Luc might have better advice Im just trying to relay what i have learnt from Lucs videos but im still a novice trader