If they are pausing with the intent to fix it because they can tell it doesn't 'look right,' I let them work through it and fix it. If they are trudging onward and finishing it confidently, but it is misspelled, I stop them, have them erase it, and have them study the card to write it again.

I just wanted to mention, with my ds who had speech difficulties early on in life, I would have him do the words on markerboard twice on the first day, as well as twice (as it is assigned) on the second day. This little extra help seemed to help. Also, the words being written on index cards in black, one to a card, worked well for him. Likewise, writing in a black dry erase marker on a white marker board, and erasing each word as he completed it so each time he had a blank 'slate' to write on helped. I mention this because he always wanted to write his words in a list on a marker board, and I didn't let him. It just would have distracted him or would have been a 'help' he wasn't supposed to have when writing the next words.
In Christ,
Julie