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 $20.39   12.235%   $22.8847   $22.88   $22.90   $22.90  
  Purchase #2 $142.73   5.670%   $150.8228   $150.82   $150.80   $150.80  
  Purchase #3 $227.84   6.165%   $241.8863   $241.89   $241.90   $241.90  
  Purchase #4 $72.97   9.347%   $79.7905   $79.79   $79.80   $79.80  
  Purchase #5 $110.66   13.635%   $125.7485   $125.75   $125.75   $125.70  
  Purchase #6 $142.31   3.658%   $147.5157   $147.52   $147.50   $147.50  
  Purchase #7 $92.34   10.582%   $102.1114   $102.11   $102.10   $102.10  
  7 purchase totals = $809.24       $870.7599   $870.76   $870.75   $870.70