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 $35.46   12.705%   $39.9652   $39.97   $39.95   $40.00  
  Purchase #2 $30.25   4.761%   $31.6902   $31.69   $31.70   $31.70  
  Purchase #3 $180.90   5.276%   $190.4443   $190.44   $190.45   $190.40  
  Purchase #4 $80.74   4.643%   $84.4888   $84.49   $84.50   $84.50  
  Purchase #5 $19.41   3.311%   $20.0527   $20.05   $20.05   $20.10  
  Purchase #6 $144.74   7.451%   $155.5246   $155.52   $155.50   $155.50  
  Purchase #7 $141.83   12.355%   $159.3531   $159.35   $159.35   $159.40  
  7 purchase totals = $633.33       $681.5189   $681.51   $681.50   $681.60