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 $243.49   6.783%   $260.0059   $260.01   $260.00   $260.00  
  Purchase #2 $86.86   7.540%   $93.4092   $93.41   $93.40   $93.40  
  Purchase #3 $148.74   12.385%   $167.1614   $167.16   $167.15   $167.20  
  Purchase #4 $237.99   5.825%   $251.8529   $251.85   $251.85   $251.90  
  Purchase #5 $132.87   4.332%   $138.6259   $138.63   $138.65   $138.60  
  Purchase #6 $58.25   11.219%   $64.7851   $64.79   $64.80   $64.80  
  Purchase #7 $5.68   5.265%   $5.9791   $5.98   $6.00   $6.00  
  7 purchase totals = $913.88       $981.8195   $981.83   $981.85   $981.90