Question: Should I get a racket bought in a shop restrung before I play with it?

Good question!
Ask your supplier how "old" the racket is (when was it manufactured ) Where did it come from (Asia, China?)
how long was it on the ship for?
You get the idea, you have no knowledge of when this racket was strung.
My best advise buy the frame and take it to a professional stringer you trust.
That way you know when it was strung, what string was used and what tension it was.
or take the racket to a professional stringer and ask them to test the tension for you.
(oh by the way if the stringer says they can't test a strung racket RUN AWAY!)

Hope this helps!

Daveg