also, dvd shrink does not force you to purchase nero. you can use DVD Decrypter or ImgBurn to burn through Shrink as well (both of those programs are free)
ans JayC is right. as long as your firmware is up to date, the best results are obtained burning at or near the rated speed of the blank media. 8x discs probably shouldn't be burned any slower than 4x, 16x no slower than 8x. your rsults might vary slightly on this, but it's a good general rule.
burning too slow can cause just as many problems as burning too fast.