At least if you're a digital artist, using digital tools. And no, I'm not talking about Their Majesty Undo, blessed be their soul. I'm referring to the ability to, in an instant, flip, rotate and otherwise transform the image around. While there's obvious utility in being about to move components of your image around at a whim, there's a not-so-obvious benefit in the change of perspective that it provides.
Observe:
Okay. That is looking okay, but there's something off about it. There's something odd going on here, but I can't quite put my finger on it. Hmm...what if I rotate it a bit?
...
OH. OOOOH. Well, it seems like I found my problem.
And this is why I like transformations. So much, in fact, that I have given Sir Flip Horizontal a button of honour right next to His Majesty Undo. Flipping an image only takes a few moments and may very well reveal several of your problems.