You generally don't need a comma after "whereas" if it's followed by a dependent clause.
*Example: "She prefers to stay home, whereas he enjoys going out."
However, if "whereas" is followed by an independent clause, you do need a comma.
*Example: "I like to eat my meals quickly, whereas she likes to savor each bite."
Here's a simple way to remember:
* Dependent clause after "whereas": No comma.
* Independent clause after "whereas": Comma.
Let me know if you'd like more examples!