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 $17.74   12.597%   $19.9747   $19.97   $19.95   $20.00  
  Purchase #2 $105.38   9.165%   $115.0381   $115.04   $115.05   $115.00  
  Purchase #3 $102.44   6.331%   $108.9255   $108.93   $108.95   $108.90  
  Purchase #4 $158.49   4.268%   $165.2544   $165.25   $165.25   $165.30  
  Purchase #5 $192.27   5.120%   $202.1142   $202.11   $202.10   $202.10  
  Purchase #6 $125.51   7.517%   $134.9446   $134.94   $134.95   $134.90  
  Purchase #7 $212.86   10.251%   $234.6803   $234.68   $234.70   $234.70  
  7 purchase totals = $914.69       $980.9318   $980.92   $980.95   $980.90