First and foremost a bit of decent media and a nice burner and software for it go a long way, far more so than simply dropping the speed.
This being said speed can affect things although the effects are somewhat overhyped, although if you are dealing with junk readers aka the PS2 a more mellow burning speed is good.
I would strongly advise against burning faster than the disc is rated though. ..
.. Oh an my favourite burning software at this time is imgburn (it also has iso creation and other burning options nowadays):
http://www.free-codecs.com/download/ImgBurn.htm
I completely agree with this. Burning at slower speeds used to be one of things that was "almost always a good idea" but has now become much more of a "sometimes a good idea".
This is certainly true if burning with a decent burner with good (usually just means up to date) firmware, and using good quality blank media.
My BENQ DW1640 plus some TAIYO YUDEN DVD+R media will burn at full speed 16X and read in anything I try it in.
**edit**
I meant to mention that I too think imgburn is the bomb for ISO burning. I use it for all my PS2/XBOX/GCN ISO burning.
The PS2 will have problems reading at 8X though