Always, always add a bit of air to the innertube before installing it.

A "plumped" tube is actually quite hard to get folded, pinched, bunched or twisted as compared to installing a flattened tube, and I've seen more than enough cases of a trapped tube resulting in a dislodged tire bead.

I actually prefer to install the tube in the tire before even fitting it to the rim, but putting the tube after one bead is in the rim can work as well if some care is taken to prevent twisting it as it gets stuffed in.

Note that some air may need to be bled out of the tube as the last portion of the tire bead is being worked into the rim, this allows the tire beads to settle into the center of the rim where more slack in the beads can be created.

The valve should always be pushed into the rim just prior to inflation, and the tire moved about the rim sufficient to make the valve point to the hub center, again before inflation.

The Araya rims are somewhat notorious for not trapping the tire bead very effectively upon full inflation (made worse by aged tires that have less in the way of supple/grippy rubber covering the tire beads).