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 $227.88   11.454%   $253.9814   $253.98   $254.00   $254.00  
  Purchase #2 $211.65   5.288%   $222.8421   $222.84   $222.85   $222.80  
  Purchase #3 $117.39   4.811%   $123.0376   $123.04   $123.05   $123.00  
  Purchase #4 $57.46   5.288%   $60.4985   $60.50   $60.50   $60.50  
  Purchase #5 $71.83   7.278%   $77.0578   $77.06   $77.05   $77.10  
  Purchase #6 $199.81   4.207%   $208.216   $208.22   $208.20   $208.20  
  Purchase #7 $32.80   4.757%   $34.3603   $34.36   $34.35   $34.40  
  7 purchase totals = $918.82       $979.9937   $980.00   $980.00   $980.00