I think the lower Seattle picks, the higher the odds of a trade down. In most drafts you see the cream of the crop dry out in the late teens to early 20s. Consider some of the notable players that went in the 20-25 range in recent years:
2012: Kendall Wright (good rookie WR), Chandler Jones (#2 in rookie sacks), David DeCastro (not yet established, but was highly touted), Dont'a Hightower (very good 3-4 ILB)
2011: Adrian Clayborn (outstanding rookie season, IR'd in 2012), Phil Taylor (difference maker at DT for Cleveland), Cameron Jordan (DT/DE hybrid that has 8 sacks and counting this season)... (James Carpenter was #25)
2010: Kareem Jackson (4 ints and 13 PD's so far in 2012), Jermaine Gresham (2011 pro-bowler), Bryan Bulaga (33 starts and counting), Demarius Thomas (1210 yards, 8 TDs this season so far), Dez Bryant (already has 2500+ receiving yards in under 3 seasons), Tim Tebow (lol).
After #25, there have been a few nice picks (Devin McCourty, Doug Martin), but it seems like every year there is a bit of a talent cliff somewhere in the early to mid 20s.
I think Seattle will appreciate that, and if they are picking say 22nd, 23rd, or 24th, there is a good chance that a value too good to pass up could prevent a trade down. If they are picking in the thirties, then yes, a trade down would make a lot of sense because the talent pool is comparable between the very late 1st and early 2nd in most draft years.