I think the amount of twist you get with each base pair isnt quite the same and if you made a strand of all G you woild find the DNA doesnt coil and pack properly.

that's exactly correct. it's not just twist, it's also tilt and roll (the other parameters that describe a curved DNA helix's orientation). However, it's not really completely correct- usually you don't end up with a true permanent change in the DNA, but the DNA might be more flexible or prone to adopting certain positions for long times.