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 $246.93   8.812%   $268.6895   $268.69   $268.70   $268.70  
  Purchase #2 $199.85   12.502%   $224.8352   $224.84   $224.85   $224.80  
  Purchase #3 $59.29   13.314%   $67.1839   $67.18   $67.20   $67.20  
  Purchase #4 $50.34   3.739%   $52.2222   $52.22   $52.20   $52.20  
  Purchase #5 $75.51   9.730%   $82.8571   $82.86   $82.85   $82.90  
  Purchase #6 $65.53   4.209%   $68.2882   $68.29   $68.30   $68.30  
  Purchase #7 $204.35   5.577%   $215.7466   $215.75   $215.75   $215.70  
  7 purchase totals = $901.80       $979.8227   $979.83   $979.85   $979.80