If the tests are different, they represent different experiences & challenges to kids. My kids elect to do them, usually one in fall and one in spring. They like it.
I would wait to do SCAT again till 4th grade (different test) but would try EXPLORE if you child is mature enough to sit through 2 hrs. DS8 did EXPLORE last yr and will do it again this year (I think). Registration through talent search is well before winter (sorry, Grinity)--say Sept or Oct at the latest for Jan or Feb test dates (there are only two dates if you go through a talent search, but you need to pay the talent search fee and get accepted into that before testing--at least that's how it works here).
In 6th grade, consider ACT and SAT. Some wait till 7th grade.
