If you're going to just slice and eat your cucumbers as-is without adding anything else to them other than salt and pepper or vinegar, or tossing on top of a green salad that will be eaten quickly, then do just that - slice and eat. Since they won't be sitting in any other ingredients that will get soggy from the extra water, there's no harm to be done.
If, however, you will be making a cucumber salad or anything where they will be combined with other ingredients in a sauce or dressing of any kind, you might want to drain them first. If the dish won't be eaten within an hour or so, the water content of the cucumbers will drastically change the consistency of the dressing, making it appear curdled, or at least watered down. It's not something I would find appealing, and certainly wouldn't want that unwelcome surprise if serving guests. Cucumber sandwiches will also get soggy quickly, as the bread would soak up the water, so make sure to drain first before making finger sandwiches, or your appetizers won't be very appetizing.
So how do you drain cucumbers? It's very simple. Peel (if desired) and slice your cucumbers, then layer the slices in a colander, sprinkle salt over them, and let them sit. You won't see the water come pouring out from the cucumber like you would when emptying a can of vegetables, but it will drain slowly - 20 to 30 minutes should be sufficient. When done, you can layer the slices on a sheet of paper towels and pat lightly if you want.
Will adding salt to the cucumber alter the dish you're preparing? Not significantly. Most of the salt will drain out with the water, but I always taste-test as I'm cooking, so would know whether my cucumber salad needed less salt than usual.
Draining cucumbers does take a few minutes, but the process itself couldn't be easier. If you set them up to drain while fixing the rest of your meal, it won't add any prep time at all other than the minute to peel and slice.
~ Marie Anne