It is tradition that Chinese calligraphy be done all at once, with no corrections or reworking of the characters. This is important because the initial marks of the brush show the state of mind of the calligrapher at that exact moment. In fact, it is practically impossible to do otherwise with traditional materials as any retouching shows as such. If a mistake is made it is necessary to start over. Some people say that seal carving should be the same. While I agree with this as an ideal, even the best seal carvers were known to occasionally (or even frequently) stamp an impression of their newly completed seal and then rework it. The more experience you gain in carving seals, the less this might be necessary. But don’t feel that if your idea doesn’t work out you can’t tweak it. The problem though with carving seals is that you can always cut away more of the stone but you can’t put it back once it is removed.