I got a new sail right before a reggata and I only had a one-digit sail number! It isn't that important but it is deffinately a good thing to have the correct sail number so you don't get confused between yours and another.
You should use your class number from the builder's plaque on your boat if you want to race because it doesn't take much time or much money and will save your RC a lot of hassle. The chances are that you have bought your boat from a local sailor or you have sold a boat with sail to a local sailor. These circumstances mean that there are sails with the same numbers on being used by different people but because sales usually occur within local areas, you will probably end up racing that other sail at either your own club or local open event/regatta. Sailing is largely a self regulated sport. If you are on your own out in front or out behind a fleet and break RRS42, who is going to protest you. Also, who is going to measure your hull at club level to make sure you haven't added or taken away bits?? It is an ILCA class rule that states that you MUST display the hull's or your own laser's sail number on the sail. If you break this rule, how many others do you plan to break before someone protests you?
Also: your RC is probably a volunteer and almost puts more effort into running your racing than you ever do so do him/her a favour. Most RC's have enough trouble working out which boats are standards and which are radials so don't add the hassle of making him/her spot the difference between boats with no difference except a 3mm by 15mm number on the inside edge of the cockpit - I guarentee that you will be sitting so his/her binoculars cannot see that.