Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $125.78   10.791%   $139.3529   $139.35   $139.35   $139.40  
  Purchase #2 $32.95   7.815%   $35.525   $35.53   $35.55   $35.50  
  Purchase #3 $62.66   6.717%   $66.8689   $66.87   $66.85   $66.90  
  Purchase #4 $109.68   5.573%   $115.7925   $115.79   $115.80   $115.80  
  Purchase #5 $238.71   4.795%   $250.1561   $250.16   $250.15   $250.20  
  Purchase #6 $149.26   4.138%   $155.4364   $155.44   $155.45   $155.40  
  Purchase #7 $184.74   9.787%   $202.8205   $202.82   $202.80   $202.80  
  7 purchase totals = $903.78       $965.9523   $965.96   $965.95   $966.00