The Safeco Field bleachers are located in sections 180-195 in left and center field. The main video board is directly above center field seating so you will want to avoid those if you want a direct look at the big board.
Both team bullpens are in front of Sections 183-185 and the first row of 182 will occasionally see a home run ball from a right-handed hitter. All of these seats are actually in the second deck of seating and the seats in CF especially are very far away from home plate. The CF bleachers are also a poor option for avoiding the sun. It's no surprise that the center field bleachers are usually the cheapest tickets in Safeco Field.
With the section tunnel entrance is located at Row 6, so even the walk up to Row 17 won't be too bad.