For flat kite, you normally don't need a safety leash in most conditions
as you can use the chicken loop as the safety leash.
However, in way overpowered situation (at about 10% of the top end range
of the kite), a safety leash is needed as the kite still has quite a bit
of power even when you let you of the bar.
So make sure you have at least a safety handle on your bar and better
yet, wear a safety leash on your harness and clip it on the safety
handle whenever you has to pull the trim strap more than 1/2 way.
A beginner should always use a safety leash even with flat kite.