There have been cases where F1 students under Optional Practical Training (OPT) status faced difficulties while re-entry into the US. It is extremely important to have valid documentation verifying your eligibility for re-entry. Please be advised that if you do not have proof of employment/job offer prior to departure, there is no guarantee that the immigration officer will allow you back in the States. Following is the info graphic that shows the risk level while traveling on OPT. If your situation is risky, it is recommended that you do not travel outside US unless absolutely necessary.