8x scan speed is the standard that the majority of us use here, so it helps us to compare apples to apples.
Burning slower on most media will produce a better burn to some degree, however, a 16x burner is meant to burn at speeds of 8x - 16x. If you have crap media and want to burn it slower in an attempt to get better results, you are better off burning in a 8x burner. That is why I have one 8x burner left in my system config. But, you will have to experiment with each different media when you are using off brands. If you buy the quality media like verbatim, or taiyo yuden, you can feel very confident about burning the media at it's full rated speed, and usually a little higher. 8x taiyo yuden burns very well in most burners at up to 16x, with excellent results at 12x. If I'm doing a system backup of critical files, I will burn at 8x to 12x to ensure a high quality burn, on 16x rated media.
I wouldn't be satisfied with a 90% burn myself. I would feel much more comfortable with around 94 - 95%, but I burn dvd's for customers, so I set I higher standard for my burns. If you are just doing movies and stuff for fun and personal use, 90% is fine as long as it still plays ok in your standalone. Watch for deals on fujifilm media, and made in japan media, and you will get much better results at nearly the same price.