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 $24.28   13.386%   $27.5301   $27.53   $27.55   $27.50  
  Purchase #2 $40.54   8.513%   $43.9912   $43.99   $44.00   $44.00  
  Purchase #3 $118.36   4.566%   $123.7643   $123.76   $123.75   $123.80  
  Purchase #4 $243.88   8.627%   $264.9195   $264.92   $264.90   $264.90  
  Purchase #5 $13.73   8.756%   $14.9322   $14.93   $14.95   $14.90  
  Purchase #6 $104.77   5.484%   $110.5156   $110.52   $110.50   $110.50  
  Purchase #7 $223.43   13.792%   $254.2455   $254.25   $254.25   $254.20  
  7 purchase totals = $768.99       $839.8984   $839.90   $839.90   $839.80