If there is only one row that tells you to sl st in a sl st of a previous row, it probably won't make any difference which way you do it. But for something like a snowflake, it might tell you to do that on the next row as well. In that case, I would sl st into the sl st, not the sc of the previous round, as that will make for more bulk, and it wouldn't look very nice, either.

Donna, thanks. When you say you would sl st into the sl st - do you mean into as under the two top threads of the loop.

Donna, I should add to the above question - meaning in particular when it is the previous joining sl st I am doing a sl st into.
I'm use to joining/sl st into top of chain or into first sc, etc. rather than into the joining sl st which I've now come across in directions for some patterns.

I would sl st into the top two threads of the sl st in the previous row.

Yes, I agree with Sheilajoyce. Just the top two threads.

thank you!

I hope this link works it is from Annies Attic and a short video of the slip stitch

Here is a link that might be useful: Slip Stitch video

minnie_tx - thanks. I know how to do a slip stitch, wasn't sure how to work into a joining slip stitch, the one made in the previous round.