I’d try to avoid it wherever possible. From the Catechism (emphasis my own):
2142 The second commandment prescribes respect for the Lord’s name. Like the first commandment, it belongs to the virtue of religion and more particularly it governs our use of speech in sacred matters.
2143 Among all the words of Revelation, there is one which is unique: the revealed name of God. God confides his name to those who believe in him; he reveals himself to them in his personal mystery. The gift of a name belongs to the order of trust and intimacy. **“The Lord’s name is holy.” **For this reason man must not abuse it. He must keep it in mind in silent, loving adoration. He will not introduce it into his own speech except to bless, praise, and glorify it.
2155 **The holiness of the divine name demands that we neither use it for trivial matters, **nor take an oath which on the basis of the circumstances could be interpreted as approval of an authority unjustly requiring it. When an oath is required by illegitimate civil authorities, it may be refused. It must be refused when it is required for purposes contrary to the dignity of persons or to ecclesial communion.
Psalm 29:2 also says “Bring to the Lord glory and honour: bring to the Lord glory to his name: adore ye the Lord in his holy court.”
The first point is this, that glory is not being given to God by the casual use of His name. There’s a big difference between “Help us now, o God in heaven, if it is Your will!” (or another form intended as an actual prayer) and appending casually “God willing” onto everything you say. Casual use of God’s name reduces our reverence because it ignores its glory and holiness and trivialises it to the hearer (note: not necessarily to the speaker) by bringing God down to the everyday level.
The second point is that “God willing” is often abused and can lead us into difficult territory theologically. By abuse I mean that it’s not uncommon to hear somebody say “see you in half an hour, God willing”, then after two hours they turn up saying “it was the will of God that I be late” - what they should really be saying usually is “sorry I’m late” because God has not fixed the future (i.e. daily events are not predestined) and He is not in direct control of our timekeeping (free will and all that). I’m thinking particularly of the Arabic “in sha’ Allah” here which is so trivialised and misused but the same could maybe be said of the English version.
Say somebody’s going to war and his family says “he will come back safely, God willing”, but then he’s tortured and killed by the enemy. I don’t think it’s God’s will that the soldier didn’t come back safely: true, God has permitted the soldier’s torture and death to happen by giving free will to the torturers, but the torturers have then chosen to use their free will to do something wrong. God doesn’t will the evil that people do. This is getting into a separate issue, though.
I’ve had this discussion a lot with Oriental Orthodox and Sunnis but never with other Catholics, so I’m interested to see what people think of my €0.02.