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 $227.68   11.826%   $254.6054   $254.61   $254.60   $254.60  
  Purchase #2 $110.91   5.211%   $116.6895   $116.69   $116.70   $116.70  
  Purchase #3 $125.89   8.498%   $136.5881   $136.59   $136.60   $136.60  
  Purchase #4 $198.86   3.809%   $206.4346   $206.43   $206.45   $206.40  
  Purchase #5 $24.53   9.255%   $26.8003   $26.80   $26.80   $26.80  
  Purchase #6 $89.82   12.662%   $101.193   $101.19   $101.20   $101.20  
  Purchase #7 $128.77   10.668%   $142.5072   $142.51   $142.50   $142.50  
  7 purchase totals = $906.46       $984.8181   $984.82   $984.85   $984.80