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 $86.67   6.816%   $92.5774   $92.58   $92.60   $92.60  
  Purchase #2 $29.25   11.729%   $32.6807   $32.68   $32.70   $32.70  
  Purchase #3 $182.27   5.673%   $192.6102   $192.61   $192.60   $192.60  
  Purchase #4 $156.76   8.867%   $170.6599   $170.66   $170.65   $170.70  
  Purchase #5 $51.80   8.404%   $56.1533   $56.15   $56.15   $56.20  
  Purchase #6 $64.57   11.522%   $72.0098   $72.01   $72.00   $72.00  
  Purchase #7 $22.66   9.752%   $24.8698   $24.87   $24.85   $24.90  
  7 purchase totals = $593.98       $641.5611   $641.56   $641.55   $641.70