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 $75.99   6.557%   $80.9727   $80.97   $80.95   $81.00  
  Purchase #2 $27.38   12.865%   $30.9024   $30.90   $30.90   $30.90  
  Purchase #3 $42.83   8.188%   $46.3369   $46.34   $46.35   $46.30  
  Purchase #4 $75.38   10.514%   $83.3055   $83.31   $83.30   $83.30  
  Purchase #5 $177.47   7.162%   $190.1804   $190.18   $190.20   $190.20  
  Purchase #6 $20.59   7.395%   $22.1126   $22.11   $22.10   $22.10  
  Purchase #7 $174.45   6.688%   $186.1172   $186.12   $186.10   $186.10  
  7 purchase totals = $594.09       $639.9277   $639.93   $639.90   $639.90