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 $238.35   10.229%   $262.7308   $262.73   $262.75   $262.70  
  Purchase #2 $221.65   8.567%   $240.6388   $240.64   $240.65   $240.60  
  Purchase #3 $125.26   7.358%   $134.4766   $134.48   $134.50   $134.50  
  Purchase #4 $152.63   12.711%   $172.0308   $172.03   $172.05   $172.00  
  Purchase #5 $148.83   4.355%   $155.3115   $155.31   $155.30   $155.30  
  Purchase #6 $85.38   10.334%   $94.2032   $94.20   $94.20   $94.20  
  Purchase #7 $234.90   6.427%   $249.997   $250.00   $250.00   $250.00  
  7 purchase totals = $1,207.00       $1,309.3887   $1,309.39   $1,309.45   $1,309.30