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 $18.32   12.332%   $20.5792   $20.58   $20.60   $20.60  
  Purchase #2 $130.78   3.125%   $134.8669   $134.87   $134.85   $134.90  
  Purchase #3 $129.61   13.556%   $147.1799   $147.18   $147.20   $147.20  
  Purchase #4 $225.84   5.633%   $238.5616   $238.56   $238.55   $238.60  
  Purchase #5 $138.56   13.422%   $157.1575   $157.16   $157.15   $157.20  
  Purchase #6 $117.75   8.863%   $128.1862   $128.19   $128.20   $128.20  
  Purchase #7 $159.75   6.474%   $170.0922   $170.09   $170.10   $170.10  
  7 purchase totals = $920.61       $996.6235   $996.63   $996.65   $996.80