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 $17.89   9.294%   $19.5527   $19.55   $19.55   $19.60  
  Purchase #2 $9.77   11.641%   $10.9073   $10.91   $10.90   $10.90  
  Purchase #3 $104.50   5.531%   $110.2799   $110.28   $110.30   $110.30  
  Purchase #4 $58.88   12.403%   $66.1829   $66.18   $66.20   $66.20  
  Purchase #5 $87.31   4.362%   $91.1185   $91.12   $91.10   $91.10  
  Purchase #6 $111.31   3.498%   $115.2036   $115.20   $115.20   $115.20  
  Purchase #7 $7.65   7.156%   $8.1974   $8.20   $8.20   $8.20  
  7 purchase totals = $397.31       $421.4423   $421.44   $421.45   $421.50