As MaxWong said, while solving the integral it does not matter which structure you use. But for convenience, it is better to make it look like
e−(s−a)t for two reasons. The first reason is that the transformation answer will involve the expression
(s−a) rather than
(a−s). The second reason is that it is some kind of a reminder to tell you that
s>a.
Another way to see this is to look at this function,
e−at. The convenient structure will be
e−(s+a)t because the answer is
s+a1 which involves the expression
(s+a).