If they are unparked you can SEO to your heart's content. If parked you are violating every parking co's TOS. We have found situations (double digits out of thousands of parked names) where rogue backlinks (unrequested) are added to parked names (typically directory listings but also others) or added months after the SEO work ended. In a few cases (certainly not all) G has found the new backlink(s) on the parked name and fail listed the name - this can and does happen enough that it is a "thing" to be aware of. You will increase the likelihood of this happening by building links on a name you intend to park. We no longer unpark/do SEO/repark domains as a result. You are likely much more skilled than we are so would be interested to read about your results.