We have about 25 users here and doing a samba restart doesn't seem to affect anything, except for an accounting program (MYOB) that continually updates to a database located on a samba share.
Anyway samba has a 'reload' option which just reloads the smb.conf file without dropping everything. On most systems this is
or you can do it manually be sending a SIGHUP to the samba process
kill -s SIGHUP `cat /var/run/smbd.pid`