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 $134.42   10.531%   $148.5758   $148.58   $148.60   $148.60  
  Purchase #2 $29.51   3.804%   $30.6326   $30.63   $30.65   $30.60  
  Purchase #3 $181.41   4.721%   $189.9744   $189.97   $189.95   $190.00  
  Purchase #4 $120.57   11.366%   $134.274   $134.27   $134.25   $134.30  
  Purchase #5 $143.67   5.177%   $151.1078   $151.11   $151.10   $151.10  
  Purchase #6 $52.93   5.294%   $55.7321   $55.73   $55.75   $55.70  
  Purchase #7 $187.49   8.498%   $203.4229   $203.42   $203.40   $203.40  
  7 purchase totals = $850.00       $913.7196   $913.71   $913.70   $913.70