As a general rule, I throw a 
block bet against very 
tight players, with the intention of folding to a 
raise, and I 
check call against very aggressive players.
Some would argue that the 
check call is 
spew with a 
draw heavy board, but that has not been my experience at all at anything above .50/1 
level.  You get value by inducing medium to very aggressive players to 
bluff when these players would have likely folded to a bet by you.  Of course you lose money when they hit, but I find that many aware players will only 
value bet small w/ a made 
river flush anyway, especially when you are showing weakness.  Frequently this 
value bet is equal to or only slightly more than your 
blocking bet would have been.
Of course, the amount I'
m willing to 
call depends on the aggressiveness of the player.  Also, I am 
check folding here when I simply can't put the player on anything else but the 
draw.