if you string the mains, stop for an extended amount of time and then come back to finish the crosses, you deserve to get kicked in the nuts.
Every time you tension a string, the racquet gets distorted because of the various forces acting upon it. While modern racquet are built to survive nuclear fallouts and Marat Safin's abuse (alright... most of his abuses), leaving a racquet with only the mains tensioned on the mount decreases the lifespan of the racquet significantly.
Will you or your customers notice? Possibly, depending on the racquet and the person. If it is a person who players once a month and strings their racquet once a year, then the racquet will never go through enough stringing and flexes to render the racquet "dead".
So basically, finish stringing what you started as soon as possible.
Volkl C10 Pro w/ Fairway Leather Grip
Babolat Star 3
Innocent! 6 13 45