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 $124.92   11.704%   $139.5406   $139.54   $139.55   $139.50  
  Purchase #2 $246.64   11.738%   $275.5906   $275.59   $275.60   $275.60  
  Purchase #3 $45.99   5.286%   $48.421   $48.42   $48.40   $48.40  
  Purchase #4 $37.25   5.707%   $39.3759   $39.38   $39.40   $39.40  
  Purchase #5 $211.51   13.212%   $239.4547   $239.45   $239.45   $239.50  
  Purchase #6 $18.44   4.770%   $19.3196   $19.32   $19.30   $19.30  
  Purchase #7 $14.99   4.312%   $15.6364   $15.64   $15.65   $15.60  
  7 purchase totals = $699.74       $777.3388   $777.34   $777.35   $777.30