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 $231.63   11.781%   $258.9183   $258.92   $258.90   $258.90  
  Purchase #2 $11.86   7.357%   $12.7325   $12.73   $12.75   $12.70  
  Purchase #3 $211.78   3.234%   $218.629   $218.63   $218.65   $218.60  
  Purchase #4 $88.69   5.660%   $93.7099   $93.71   $93.70   $93.70  
  Purchase #5 $62.36   9.639%   $68.3709   $68.37   $68.35   $68.40  
  Purchase #6 $32.54   3.200%   $33.5813   $33.58   $33.60   $33.60  
  Purchase #7 $183.86   7.850%   $198.293   $198.29   $198.30   $198.30  
  7 purchase totals = $822.72       $884.2349   $884.23   $884.25   $884.20