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 $83.25   12.119%   $93.3391   $93.34   $93.35   $93.30  
  Purchase #2 $132.65   4.698%   $138.8819   $138.88   $138.90   $138.90  
  Purchase #3 $194.51   3.279%   $200.888   $200.89   $200.90   $200.90  
  Purchase #4 $153.50   3.625%   $159.0644   $159.06   $159.05   $159.10  
  Purchase #5 $26.86   7.198%   $28.7934   $28.79   $28.80   $28.80  
  Purchase #6 $182.71   12.750%   $206.0055   $206.01   $206.00   $206.00  
  Purchase #7 $141.89   10.666%   $157.024   $157.02   $157.00   $157.00  
  7 purchase totals = $915.37       $983.9963   $983.99   $984.00   $984.00