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 $60.98   6.114%   $64.7083   $64.71   $64.70   $64.70  
  Purchase #2 $86.88   9.266%   $94.9303   $94.93   $94.95   $94.90  
  Purchase #3 $74.38   7.478%   $79.9421   $79.94   $79.95   $79.90  
  Purchase #4 $243.66   8.430%   $264.2005   $264.20   $264.20   $264.20  
  Purchase #5 $23.94   12.786%   $27.001   $27.00   $27.00   $27.00  
  Purchase #6 $128.76   8.589%   $139.8192   $139.82   $139.80   $139.80  
  Purchase #7 $140.47   4.691%   $147.0594   $147.06   $147.05   $147.10  
  7 purchase totals = $759.07       $817.6608   $817.66   $817.65   $817.60