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 $228.62   12.182%   $256.4705   $256.47   $256.45   $256.50  
  Purchase #2 $175.57   3.645%   $181.9695   $181.97   $181.95   $182.00  
  Purchase #3 $165.68   7.241%   $177.6769   $177.68   $177.70   $177.70  
  Purchase #4 $62.82   6.197%   $66.713   $66.71   $66.70   $66.70  
  Purchase #5 $14.86   7.743%   $16.0106   $16.01   $16.00   $16.00  
  Purchase #6 $197.90   8.170%   $214.0684   $214.07   $214.05   $214.10  
  Purchase #7 $55.66   12.112%   $62.4015   $62.40   $62.40   $62.40  
  7 purchase totals = $901.11       $975.3104   $975.31   $975.25   $975.40