Well, I have done it both ways, but prefer to put extra strand underneath. Never had a problem, like Bear, Steve mentions.
Either by hand or machine, never a problem with the roundness of serving. If you lay the strands out, work them over with some leather then serve.
If you need alittle help, use a pair of Hemastats or clothes pins to hold the ends while serving.
You can also mark the area where serving, unstring the bow, and lay the strands in the string, then restring, and serve.