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 $86.84   11.308%   $96.6599   $96.66   $96.65   $96.70  
  Purchase #2 $101.91   8.544%   $110.6172   $110.62   $110.60   $110.60  
  Purchase #3 $239.32   7.872%   $258.1593   $258.16   $258.15   $258.20  
  Purchase #4 $125.96   8.257%   $136.3605   $136.36   $136.35   $136.40  
  Purchase #5 $53.25   10.553%   $58.8695   $58.87   $58.85   $58.90  
  Purchase #6 $60.38   10.719%   $66.8521   $66.85   $66.85   $66.90  
  Purchase #7 $43.73   13.430%   $49.6029   $49.60   $49.60   $49.60  
  7 purchase totals = $711.39       $777.1214   $777.12   $777.05   $777.30