It is never okay to lie, and it's never okay to tell your child to lie. There is no such thing as a "little white lie" and God clearly tells us this.
When someone asks you, for example, "Do you like my new dress?" and you find it hideous, you can always look to find one thing nice to say, such as "It's a beautiful color." No further words are needed.
Tact and lying are really two different things, and while it does take a little practice to perfect tact, it can be done (and more people ought to do it.)
***ETA*** Now that I've read a few of your answers, Wild Woman hit the nail on the head and gave a perfect illustration of the point I was trying to make...way to go! ;)
(Also...to say that ones opinion isn't truth...oh yes, it most certainly is. How "x" feels about something may not be a universal truth...as in not EVERYONE feels that way...but for "x" to say, "I like it" when he doesn't is an absolute lie and it has nothing to do with opinion. When you have an opinion, speaking it as it is is the truth. To say something you don't believe is a lie.)