Go back and undo what needs to be fixed now. It's a good habit to start doing. Lay quilt top on table, back side up, place a safety pin where the seam needs to be fixed.

Fix the seams now. I was given several quilts with seams like that and when I did a slight pull on each row, they literally came apart.. and the quilts were already quilted. Sad state of affairs as I don't have the time, or desire, to pick out LA quilting and then undue and repair the problem. Good fabric was used, LA quilting great, but quilt will end up in a dog shelter for a blanket.

No need to do anything. 1/8 inch seams will hold up fine, especially if you will be quilting over them. The only reason to change them would be if you have some very loosely woven fabric.
When I make my scrap quilts, all the seams are 1/8 inch.

It it is entire seams or if it makes the blocks fit wonky I'd do over. Even though I'm an open presser in this sort of thing I just do a new seam and fold over.

But like Maviskw says, if the fabrics are fitting together and well woven and not fraying I wouldn't worry my head too much about it -- for an 8th. Anything less than that and yeah, I get worried.

I'm not very impressed with the workmanship of one of my current projects. I have a sewing superstition about doing all of a project on one machine and had to switch machines. The transition as I was getting used to the different machine shows on the backside but nothing I think that will really impact the finished project.

Are you using a tight stitch? That also makes a difference. I use a smallish stitch and sometimes that can cause more problems than I solve if I try to take it out. Plus I have vision issues so whenever possible I just cut a new piece and go on.

Like some of the others said an eighth of an inch is probably ok. I have been known to go over a seam with smaller stitches to make sure it wont unravel when washed and all my quilts are washed after I finish them and so far those seams are fine. If it isnt going to mess up your block I would take it out and redo but when it is near impossible to do that I do the tiny stitches over those seams.