Cutting hair extensions can be tricky business. It might look great after the first application, but the challenge is that if they are cut to customize, then they will need to be put back in exactly the same way when replacement time comes. That is not an easy task, even for the most experienced stylist.
A better option is to have them put in the first time in such a way that they need very little if any cutting.
While this isn't always possible, it is the preferred option. When hair extensions are cut, aside from cutting perimeter length, some of the integrity of the hair is sacrificed. After all, we are trying to add hair, not take it away. Face framing is one exception as these are easier to reinstall in the same place; length cutting should be done carefully as (obviously) the extensions don't grow, so to add more length later requires buying more hair or using more from reserve which increases your cost. When in doubt, try wearing the hair for a few days before deciding to cut the length.
If the hair needs cutting to blend and match the natural hair, a razor cut is very effective at taking out just enough hair to seamlessly blend in the layers of the natural hair. With tape-in extensions you can add hair higher up the head to get more blending of the natural hair, as they sit flatter and are less visible than some other methods. This is a great advantage over some of the other methods out there.
As always, consult your stylist for the best options of hair extension length, application and color to best suit your needs.