It's impossible to design a coaster without any lateral forces (sure maybe one with just one seat). There is always variation between all the seats on a train. Most coasters produce lateral force peaks between -1 and +1 g's, sure they all aim at 0 but no coaster that I know of produce 0 g's lateral at all times (in theory rides like Superman: Escape from Krypton, may come close to it, but there are always imperfections).
Some rides like El Toro at Plohn have turns that produce 1 g in some corners for "fun", but those are mainly wooden coasters and wild mouses, but can be used on other coasters as well, badly shaped elements on real coasters easily produces lateral forces over 4 g's, it all depends on what you are looking for.
So aim for 0 g's when designing coasters in NL, but if there are peaks up to +-1 g's then it's not the end of the world (even though some people think that way).