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 $125.45   8.391%   $135.9765   $135.98   $136.00   $136.00  
  Purchase #2 $136.76   3.839%   $142.0102   $142.01   $142.00   $142.00  
  Purchase #3 $36.95   13.291%   $41.861   $41.86   $41.85   $41.90  
  Purchase #4 $219.74   4.419%   $229.4503   $229.45   $229.45   $229.50  
  Purchase #5 $68.68   6.155%   $72.9073   $72.91   $72.90   $72.90  
  Purchase #6 $85.70   9.248%   $93.6255   $93.63   $93.65   $93.60  
  Purchase #7 $99.98   4.143%   $104.1222   $104.12   $104.10   $104.10  
  7 purchase totals = $773.26       $819.9530   $819.96   $819.95   $820.00