As a general rule you should always stay inside the confines of the lifttruck operator compartment. The machine is designed in such a way to help keep the operator protected in case of falling objects or in rare cases a tipover. By trying to exit the machine during a tipover or mishap you have no idea of how the machine is going to land and by nature it is instinct to want to get clear of a tipping over machine but your only option is to try and leave the compartment in the same direction it is falling because gravity and physics make it impossible to climb out the top side when it is tipping. In almost EVERY case i have seen the operator's chances of successfully clearing the machine is very low and it ultimately ends in severe injury or death due to the machine crushing the operator between the lift and ground.
Now being a reach truck and the operator compartment is more open and the chances of the lift tipping over backwards is very low i suppose it is possible to clear the machine if it tips over sideways or forward, but you have no idea of what is going to happen with other circumstances of what happens when the machine does tip and lands, if in racks the racks may get knocked down or any other external objects that may get knocked around.
An operators best bet is to stay put and hang on with only minimal risk of injury and eliminating the possibility of being crushed by the machine.
Now being that the redesign to alot of reach machines now a days that do include a seat, i would think that the intent is to keep you in place aside from comfort and ease of operation. So with a standup style reach truck i think it could go either way, but in the case of todays machines that include the sitdown environment the intent is to stay in place.