On the 'new invoice' page, run this JS to round hours.

freshbooks_round_hours.js

var nodes = document.getElementsByName("qty[]");
var index;
for (index = 0; index < nodes.length; ++index) {
	var node = nodes[index];
	if (node.getAttribute("title") == "Task hours") {
		var nonRoundedValueAsString = node.getAttribute("value");
		var nonRoundedValue = Number(nonRoundedValueAsString);
		var roundedValue = (Math.round(nonRoundedValue * 4) / 4).toFixed(2);
		console.log(nonRoundedValue + " => " + roundedValue);
		node.value = roundedValue.toString();
	}
}