If you want things to fit correctly, you do need to flute and bend the rib flanges as close to 90 degrees as possible.
It can be done with a little patience and everything will fit a lot nicer later on. Trying to force things in to position is a recipe for disaster, or at the least, a poor fit and problems that will be visible from the outside of the skin.
Here's a good sequence for getting the job done:
Deburr all of the edges, especially all the tabs around the nose of the rib. If you do it later, you might mess up all of that bending and fluting you did.
Make sure you have a FLAT surface, like maybe a piece of glass, to set the rib on while you are doing the work. You'll need a fluting tool and a seamer, as well.
Begin by fluting the flanges to get them as close to 90 degrees and the rib as close to flat as you can. As you go along, lay it on the glass, flanges up, to verify how flat the rib really is.
Keep at it...if you flute an area too much and the rib bends up a little, just take a seamer and squeeze the flute back out a bit and the rib will relax.
Patience is the key here and it will take a while; however, a good job here will make for a better fit in the wing, it will go easier during assembly and it will look a lot nicer from the outside after the job is done.
Good luck.