You can get it to snap on mouse release by adding a listener to the basepane of the seekbar:
B4X:
Dim Jo As JavaObject = B4XSeekBar1.mBase
Dim O As Object = Jo.CreateEventFromUI("javafx.event.EventHandler","Seekbar",Null)
Jo.RunMethod("setOnMouseReleased",Array(O))
Then adding Jorge's code to the mouse released event callback:
B4X:
Sub SeekBar_Event (MethodName As String, Args() As Object) As Object
B4XSeekBar1.Value = Round (B4XSeekBar1.Value/10) * 10
Log("Value: " & B4XSeekBar1.Value)
End Sub
Not much different but it's a little nicer experience maybe.