Should i wait for retest while trading flags?
What does your historical testing show?
What does your live trade journal show?
I do not believe anyone can answer this because only you know your trading system.
I was a big fan of such chart patterns and if I remember it correctly flag and pennant were such patterns which led to a quick continuation. In case of a triangle you can wait for a pullback but flag - no. The slope of the trendline is also important.
But that was 10 years ago so you have to check how it is now 
good luck