Both the prosecution and defense attempt to strategize for best jurors, Is this considered jury tampering?
No, this is not jury tampering. Jury tampering is an offense in which someone unduly or illegally tries to influence a jury's composition or the decision that it comes to in a trial. The important words here are "unduly" and "illegally."
Of course, lawyers from both sides are trying to influence the jury at every moment of a trial when they are in the presence of the jury. That is the whole point of having lawyers. If it were a crime simply to try to influence the jury, the legal system as we know it would not be able to exist.
As long as they follow the rules, lawyers may (and indeed must) attempt to influence the composition and decisions of the jury. The process of jury selection is very much within the rules. It is an attempt to influence the composition of the jury, but it is not "undue" or "illegal."