There are a number of avenues available. The easiest is to use the formatters (
http://code.google.com/apis/chart/interactive/docs/reference.html#formatters), which will format your data for you according to the parameters you specify. You can also manually format the output using the DataTable#setFormattedValue method or by entering values into the table as an object: {v: value, f: 'string formatted value'}. The third option is to use the 'tooltip' column role type to create your own. From what you said, though, formatters should work fine for you. Use them like this:
// format the second column as a percent
var formatter = new google.visualization.NumberFormat({
pattern: '#,###%'
});
formatter.format(data, 1);