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 $51.36   10.220%   $56.609   $56.61   $56.60   $56.60  
  Purchase #2 $46.86   5.784%   $49.5704   $49.57   $49.55   $49.60  
  Purchase #3 $121.35   12.446%   $136.4532   $136.45   $136.45   $136.50  
  Purchase #4 $64.54   9.484%   $70.661   $70.66   $70.65   $70.70  
  Purchase #5 $142.95   7.531%   $153.7156   $153.72   $153.70   $153.70  
  Purchase #6 $48.83   8.206%   $52.837   $52.84   $52.85   $52.80  
  Purchase #7 $239.55   8.588%   $260.1226   $260.12   $260.10   $260.10  
  7 purchase totals = $715.44       $779.9688   $779.97   $779.90   $780.00