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 $9.99   6.720%   $10.6613   $10.66   $10.65   $10.70  
  Purchase #2 $165.40   7.518%   $177.8348   $177.83   $177.85   $177.80  
  Purchase #3 $48.71   12.662%   $54.8777   $54.88   $54.90   $54.90  
  Purchase #4 $21.67   10.642%   $23.9761   $23.98   $24.00   $24.00  
  Purchase #5 $156.34   5.425%   $164.8214   $164.82   $164.80   $164.80  
  Purchase #6 $118.70   5.628%   $125.3804   $125.38   $125.40   $125.40  
  Purchase #7 $68.37   9.870%   $75.1181   $75.12   $75.10   $75.10  
  7 purchase totals = $589.18       $632.6698   $632.67   $632.70   $632.70