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 $128.73   7.413%   $138.2728   $138.27   $138.25   $138.30  
  Purchase #2 $28.40   8.593%   $30.8404   $30.84   $30.85   $30.80  
  Purchase #3 $221.58   10.586%   $245.0365   $245.04   $245.05   $245.00  
  Purchase #4 $84.65   9.319%   $92.5385   $92.54   $92.55   $92.50  
  Purchase #5 $163.43   4.229%   $170.3415   $170.34   $170.35   $170.30  
  Purchase #6 $21.40   8.357%   $23.1884   $23.19   $23.20   $23.20  
  Purchase #7 $122.32   9.742%   $134.2364   $134.24   $134.25   $134.20  
  7 purchase totals = $770.51       $834.4545   $834.46   $834.50   $834.30