Is swearing (making an oath) considered bad in Christianity?

There seems to be no clear understanding of James 5:12. John Wesley (Explanatory Notes on the Whole Bible) wrote:

"[The Apostle] does not forbid the taking a solemn oath before a magistrate. Let your yea be yea; and your nay, nay - Use no higher asseverations in common discourse; and let your word stand firm. Whatever ye say, take care to make it good."

And the Pulpit Commentary informs pulpiteers:

"James condemns only what is called profane swearing [profanity, and] hasty and frivolous oaths. Some commentators ..., some philosophers ..., some Fathers of the early Church, and some Christian sects ... interpret this command ... as an absolute condemnation of all kinds of swearing. [However,] upon solemn occasions oaths may be not only lawful, but sometimes also dutiful.... And Christian intelligence suggests that there can be nothing sinful in this, provided it be done only upon a solemn judicial occasion and in a reverent spirit. The words in the third commandment which are emphatic are evidently the words "in vain," it being assumed that there is a lawful use of the Divine Name."

On the other hand:

• "Swear not (mē omnuete).... The Jews were wont to split hairs in their use of profanity, and by avoiding God’s name imagine that they were not really guilty of this sin, just as professing Christians today use “pious oaths” which violate the prohibition of Jesus." ~ Robertson Word Pictures
• "But let our yea be yea - Let there be a simple affirmation, unaccompanied by any oath or appeal to God or to any of his works. A man who makes that his common method of speech is the man who will be believed." ~ Albert Barnes' Notes on the Bible (ca. 1850 A.D.)
• "When we become impatient and lose self-control we tend to say things better left unspoken. These include swearing [profanity], abusing the Lord's name, and appealing to heaven, earth, or whatever as confirmation that we are speaking the truth (cf. Matt. 5:33-37)." ~ The Expository Notes of Dr. Constable 2012
• "Because even the best men sometimes through impatience slip and speak oaths sometimes lesser, sometimes greater, the apostle warns us to detest such wickedness, and to accustom our tongues to simple and true talk." ~ Geneva Bible Translation Notes 1599

I've heard this too, but I don't know where it comes from and I've never read it in any of the Catechetical texts I use for teaching the faith. Certainly, there's a Biblical edict against swearing like that as @Narnian makes quite clear. But, there's another side to swearing. Which is making a vow.

Vows are very central to Catholicism, all of the Sacraments are essentially vows. All the covenants are vows. Everything in Catholicism is swearing, on your honor that you'll up hold your end of the bargain.

So, what's really to be avoided is an oath made in vain. Because that would be a violation of the 2nd commandment to not use the Lord's name in vain (or invoke His holy name for a pointless reason).

But, even rash vows have a place in Catholicism. Love itself, is a vow and to say, because it was entered in to on a whim, is not even vanity, it's closer to Godliness and it makes one freer as one binds themselves:

The revolt against vows has been carried in our day even to the extent of a revolt against the typical vow of marriage. It is most amusing to listen to the opponents of marriage on this subject. They appear to imagine that the ideal of constancy was a yoke mysteriously imposed on mankind by the devil, instead of being, as it is, a yoke consistently imposed by all lovers on themselves. They have invented a phrase, a phrase that is a black and white contradiction in two words—'free-love'—as if a lover ever had been, or ever could be, free. It is the nature of love to bind itself, and the institution of marriage merely paid the average man the compliment of taking him at his word.

GK Chesterton - In Defence of Rash Vows

So, if I ever find out where that teaching came from, I'll append it to here, but in general, making sacred oaths is about as taboo in Catholicism as wearing a Rosary around your neck _{(Read Manzoni's the Betrothed if you've ever had scruples about that)}.