Does the speed after exit mars orbit have to be exactly 7250m/s ?
First, you never "exit Mars orbit" for voyager quest.
You only _
pass_ the line of mars' orbit around the sun. You never need to go close to Mars itself.
Second, no, you do not need to be going exactly 7250 m/s. Just 7250 m/s or more.
7250 m/s is simply minimum escape velocity from the sun when you are as far from the Sun as Mars is.
If you are going 7249 m/s for example, you did not escape the Sun and eventually you will come back if you wait a LONG time.
If you are going at 7250 m/s or faster, you will escape the Sun and NEVER come back. This is how we know when you've succeeded.