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 $38.87   3.621%   $40.2775   $40.28   $40.30   $40.30  
  Purchase #2 $186.75   13.287%   $211.5635   $211.56   $211.55   $211.60  
  Purchase #3 $193.47   6.587%   $206.2139   $206.21   $206.20   $206.20  
  Purchase #4 $242.54   8.779%   $263.8326   $263.83   $263.85   $263.80  
  Purchase #5 $98.52   4.443%   $102.8972   $102.90   $102.90   $102.90  
  Purchase #6 $229.50   7.611%   $246.9672   $246.97   $246.95   $247.00  
  Purchase #7 $200.93   5.417%   $211.8144   $211.81   $211.80   $211.80  
  7 purchase totals = $1,190.58       $1,283.5663   $1,283.56   $1,283.55   $1,283.60