While dislocation is possible, it is rare for this to occur after shoulder arthroplasty. The authors of one paper* reported an incidence of 2.8% (5 out of 176 shoulders) for this complication. However, 4 out of 5 of the dislocating shoulders in this series of patients were associated with either a loose glenoid component or a torn rotator cuff. Thus, there was only one shoulder out of 176 that dislocated without another known associated problem.
If you follow the standard shoulder precautions the risk of dislocation or other injury will be minimized.
*Norris TR, Iannotti JP. Functional outcome after shoulder arthroplasty for primary osteoarthritis: a multicenter study. J Shoulder Elbow Surg. Mar-Apr 2002;11(2):130-135.