I have heard that even people who I regard as great bible teachers have said ‘I would be afraid not to tithe’

 I do not believe that we should be motivated by fear to give……. LOVE should be our motivation and perfect love (ie Gods love in us) casts out all fear

1Jn 4:18 'There is no fear in love; perfect love casts out all fear because fear involves torment. But he who fears has not been made perfect in love'

 What we do out of fear is not done in faith……

and according to Rom 14:23…. ‘everything that is not of faith is sin’

 It would therefore be a reasonable conclusion that tithing done out of fear is SIN….and most certainly will not be blessed by God!

 (With this in mind perhaps using Mal 3:8-12 as a sledgehammer will in future be regarded as inappropriate)