Personally, as stated before, "In God We Trust," wasn't added the currency until later in U.S. history. I don't believe it should be printed on the money because of the whole Separation of church and state business. It was present on all money in the late 1950's as part of a brain washing technique in my opinion. It's similar to "One nation, under god" in the pledge. That wasn't added until much later to gain support for the wars. I don't believe it should be there. I don't see the need, and all it does now is give current Christian Extremists an Ego boost.
It was pointed out by Valerian that it has been on US money in one form or another since the late 1800's.
It was first suggested (probably) in 1861 and added to the then brand-new two cent coin in 1864. After that it was used, off and on, on the various U.S. coins; and declared the official motto of the U.S. in 1956. The following year it was added to paper money and has been on all U.S. currency since.
Over all, it does -not- espouse a Church philosophy and those saying it violates the Church/state separation are really reaching if they think this pushes a theocratic regime. The phrase, 'In God we trust' acknowledges something mentioned in the Declaration of Independence, that our freedoms do not come from Human reasoning's or design, but from something greater than us. It might be a god or gods. Perhaps it is none of those, but the founding fathers knew that anything that humans give, we can take away. If the freedoms we enjoy in this country are Human given, then they can be taken away.
To keep them requires human diligence, against those that want to em-place their own religious philosophers and against those that want to help us 'for our own good'. Yet the freedoms are there and need to be maintained by constant vigilance from those that want to change them.