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 $192.93   5.604%   $203.7418   $203.74   $203.75   $203.70  
  Purchase #2 $120.81   11.544%   $134.7563   $134.76   $134.75   $134.80  
  Purchase #3 $163.44   9.106%   $178.3228   $178.32   $178.30   $178.30  
  Purchase #4 $27.63   6.529%   $29.434   $29.43   $29.45   $29.40  
  Purchase #5 $28.93   7.206%   $31.0147   $31.01   $31.00   $31.00  
  Purchase #6 $115.31   9.671%   $126.4616   $126.46   $126.45   $126.50  
  Purchase #7 $111.99   12.765%   $126.2855   $126.29   $126.30   $126.30  
  7 purchase totals = $761.04       $830.0167   $830.01   $830.00   $830.00