A perfectly straight hairline is generally not something that doctors suggest. Not only does it run the risk of looking really fake and unnatural if not done right, but the straight hairline that everyone wants now may not be 'fashionable' in 10 or so years, which could force many into wanting to change/undo their own personal hairline.
The hairline is part of the artistry of a transplant. FUE, FUT or RFUE the hairline should never be straight. If the hairline is too straight it appears unnatural defeating the goal of a good transplant to look natural and undetectable. I recommend you visit with a physician and discuss your goals, only the perform surgeon can give you the best answer.