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 $19.51   8.246%   $21.1188   $21.12   $21.10   $21.10  
  Purchase #2 $137.60   7.800%   $148.3328   $148.33   $148.35   $148.30  
  Purchase #3 $113.66   12.660%   $128.0494   $128.05   $128.05   $128.00  
  Purchase #4 $224.54   7.175%   $240.6507   $240.65   $240.65   $240.70  
  Purchase #5 $51.78   13.776%   $58.9132   $58.91   $58.90   $58.90  
  Purchase #6 $78.92   5.695%   $83.4145   $83.41   $83.40   $83.40  
  Purchase #7 $17.78   7.325%   $19.0824   $19.08   $19.10   $19.10  
  7 purchase totals = $643.79       $699.5618   $699.55   $699.55   $699.50