You can really do it anytime, just try and handle them carefully from their cotyledon (seedling leaves) and not the stems or true leaves. I do mine right after they get their first true leaves.

Here is a link that might be useful: potting up tomato seedlings (my blog with pictures)

Thanks so much for your response. I was worried about the number of tomato plants I have in each cup but seeing your blog helped me figure out what/when to do the transplanting. I got 4" used containers from the local nursery, cleaned them out & will start transplanting the largest plants tomorrow! Thanks for the advice & the up close visuals & instructions!

Glad you found it useful :) Best of luck.

Tomatoes I start in used sixpacs, two seeds per cell. Usually both grow and I cut one off at soil level. When 3 or 4 inches high the cell will be well rooted and I slip the plant out without root disturbance and right into a salvaged four inch nursery pot. Al