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 $123.40   7.772%   $132.9906   $132.99   $133.00   $133.00  
  Purchase #2 $35.95   9.859%   $39.4943   $39.49   $39.50   $39.50  
  Purchase #3 $30.77   12.113%   $34.4972   $34.50   $34.50   $34.50  
  Purchase #4 $156.63   12.203%   $175.7436   $175.74   $175.75   $175.70  
  Purchase #5 $173.79   9.299%   $189.9507   $189.95   $189.95   $190.00  
  Purchase #6 $79.88   11.506%   $89.071   $89.07   $89.05   $89.10  
  Purchase #7 $172.29   4.123%   $179.3935   $179.39   $179.40   $179.40  
  7 purchase totals = $772.71       $841.1409   $841.13   $841.15   $841.20