Spirals are not really meant to keep a straight line - they are best used where there is a lot of side-to-side movement. Having teeth in all directions, they don't have a shape that would help them keep moving along a line. But when I have a long, straight-ish line on a portrait cut, I turn the material so I am cutting into the blade as if it were a straight blade. I find it easier to stay on the line that way. When the odd shapes come up, I find the spirals are a big help with sideways cuts.
The biggest problem I have with spirals is on long, thin lines where one pass of the blade is too thin but the line is not wide enough to get another full cut on the other side of the line. I wind up chewing away on the 2nd cut to widen the line and it does not go smoothly.