White line when isolating object

If this is happening consistently, then something is wrong.  Does Select & Mask have Remember Settings checked?  If yes, turn that off, but look to see if you have some positive Shift Edge set.   I am not sure if you can reset Select Mask, so if that is not doable, I'd go right ahead and reset Preferences.  Like I said, you shouldn't be seeing a fringe with every selection.

If all the settings are OK, then you have various automatic fix options.

Select & Mask > Shift Edge

Select & Mask > Decontaminate Color

Layer > Matting > Defringe

Layer > Matting > Remove white (or black) fringe

Set the S&M output to New Layer with Layer Mask to give you the tools to fix the selection. 

Another option is to Ctrl click the mask to load it as  selection > Modify > Contract the selection (1 pixel?) > Invert the selection, and paint out the fringe in the layer mask with black.

Do you know you can access the old Refine Edge dialog by Shift clicking Select & Mask in the Select menu.  (You need the layer mask selected for this)

Shift edge will contract or expand the selection and adding contrast may also help.

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Kim, there is another option that may help. With the object you have selected appearing on a transparent layer, choose Layer > Matting (down at the bottom) and choose Defringe or one of the other options.

If you have an earlier version of Photoshop and Select and Mask is not available, consider this.

If quick-selecting the object produces the undesirable edge, Quick Select the non-object area and Select > Inverse the Selection and see whether that produces a better result.

If, after using Quick Selection tool, you chose Select and Mask in the Options bar, you will have the option to Contract the selection and make other fine-tuning adjustments. 

Would have to see a screenshot of what your result looks like. You may need to shift the mask inwards more than 1 pixel, or you may need to do this only in certain areas, hard to say without seeing.