Don't you just hate it when you "run out" of your favorite makeup that's housed in a tube with a brush or doe foot applicator? You know you can probably pick up a few more drops of gloss or cream but you just can't reach the remaining product with the provided wand. This happened to me before. I tried cutting the tube with the knife, but I ended up *almost* cutting off my finger and wasting the contaminated product. So yes, I wouldn't recommend that.
Here's an easier way to make the most out of your tubed makeup: simply pop out the plastic, detachable rim with a thin knife. That rim makes sure that you get exactly enough product for one application, but it also prevents the wand from scraping out the product from the sides. If your makeup is almost out, removing the rim will allow you to get the rest of the product with your wand.