Should I taste-test food before serving it?
Any halfway competent chef should indeed be tasting.  The only way to know whether you're putting up good food is to check it yourself - and you'd better be consistent if you want to continue getting paid for it.

That's not to say that all chefs do, nor that there's any one standard for how frequently to taste or what method should be used.  It's pretty obvious that using your hands to taste or double-dipping is frowned on... but shortcuts do happen when it's the middle of the rush and you have 30 things to be doing at once.

The best practice I've personally seen in a commercial kitchen was to have a large set of spoons at each station, kept with the "business end" submerged in hot water with a mild sanitizer solution.  Each chef also carried a few clean towels which they changed out copiously throughout the evening (these have a million uses, from wiping hands dry to cleaning plate rims to handling hot pan handles).  When tasting, we'd grab a spoon, tap off excess water, wipe with a clean towel if necessary, take a small taste, and discard.  This got to be as much of a habit as washing your hands after handling raw meat or wiping down your cutting board.  The used spoons would be collected and washed regularly along with used pans and so on, then returned to the line.  With a system like this there's minimal risk for contamination and relatively little waste, except for frequent washing.

At home, you can play things a bit looser, unless you like washing all of your spoons every night.  Double-dipping isn't a big concern unless you're ill, in which case you shouldn't really be cooking for people.  You're probably introducing your friends and family to just as much contamination in the form of dust and such simply by having them in your home.

EDIT: I agree with Jefromi's point in comments, so I want to emphasize this further.

Safety aside, I strongly believe that tasting is necessary to make you a better cook.  It teaches you how to make corrections on the fly and balance flavors, rather than simply following a recipe.  As a matter of fact, even if you're following a recipe, you will need to account for variation in things like produce.  Fruits vary in their flavor depending on how ripe they are, where they were grown, whether it was a good season... The vegetables that I get at my local market may be a slightly different variety from yours... Meats vary widely depending on how the animal is fed and raised, even if you're using the same cuts.  Even when using packaged ingredients, you can't guarantee complete consistency unless you use the exact brand as written in a recipe.  (And I dare say that if your recipes uses only packaged ingredients, you can do much better.)  Recipes cannot account for this sort of variation in anything but the broadest strokes.  It's up to you to balance all of this, plus the preferences of yourself and your guests.  If you're only tasting when you've finished cooking, it's probably too late to correct any problems.

Like any skill, cooking ability is improved by practice and feedback.  Tasting as you go provides you with immediate feedback on how the dish tastes and what it needs, rather than just at the very end when you sit down to eat.  Learning how flavors interact, how much seasoning is enough, and so on are fundamentals.   They're what help you get creative, go beyond cooking someone else's dishes, and start coming up with your own.

So: yes, you should taste, and you should taste often.  There are many ways to deal with safety concerns, but there is no other way to get better at fixing your mistakes before they hit the table.