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 $28.90   4.568%   $30.2202   $30.22   $30.20   $30.20  
  Purchase #2 $34.79   7.116%   $37.2657   $37.27   $37.25   $37.30  
  Purchase #3 $200.34   12.520%   $225.4226   $225.42   $225.40   $225.40  
  Purchase #4 $177.79   8.221%   $192.4061   $192.41   $192.40   $192.40  
  Purchase #5 $224.82   6.731%   $239.9526   $239.95   $239.95   $240.00  
  Purchase #6 $104.38   10.344%   $115.1771   $115.18   $115.20   $115.20  
  Purchase #7 $81.84   11.719%   $91.4308   $91.43   $91.45   $91.40  
  7 purchase totals = $852.86       $931.8751   $931.88   $931.85   $931.90