It entirely depends on the type of training imparted - is it specific to an assignment which needs a commitment of a certain time span, or the training is imparted to groom the individuals for higher roles. Also, other factors like if the training is conducted internally or by an external agency, if the individuals were deputed to a different location for training (overseas, etc.).
A right combination of the above factors will determine if you should apply a bond system for those identified/undergoing training. It is important to communicate the terms & conditions/commitments to those undergoing the training well beforehand rather than after completion of training; else, it could become a non-compliance or breach of trust.
Hope the above input is useful.