As a general rule, we preferred native, parasolid, stp, iges in that order.
Native files we would translate into parasolid by way of TransMagic. We found better results by translating the files ourselves than having the submitter translate them before submittal (isn't that a word? Spell check doesn't think so).
Parasolid is a very robust format and seldom gave us any problems.
Iges was used only as a last resort, and even then it was often easier to recreate the file than use the translation, using only key surfaces from the iges file.
I hope that helps some.
"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]