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 $104.68   10.682%   $115.8619   $115.86   $115.85   $115.90  
  Purchase #2 $196.30   8.778%   $213.5312   $213.53   $213.55   $213.50  
  Purchase #3 $163.26   8.362%   $176.9118   $176.91   $176.90   $176.90  
  Purchase #4 $134.27   10.781%   $148.7456   $148.75   $148.75   $148.70  
  Purchase #5 $201.32   4.686%   $210.7539   $210.75   $210.75   $210.80  
  Purchase #6 $19.46   6.806%   $20.7844   $20.78   $20.80   $20.80  
  Purchase #7 $127.33   6.112%   $135.1124   $135.11   $135.10   $135.10  
  7 purchase totals = $946.62       $1,021.7012   $1,021.69   $1,021.70   $1,021.70