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 $247.79   13.765%   $281.8983   $281.90   $281.90   $281.90  
  Purchase #2 $88.55   4.103%   $92.1832   $92.18   $92.20   $92.20  
  Purchase #3 $250.85   11.176%   $278.885   $278.89   $278.90   $278.90  
  Purchase #4 $163.79   13.314%   $185.597   $185.60   $185.60   $185.60  
  Purchase #5 $70.79   5.256%   $74.5107   $74.51   $74.50   $74.50  
  Purchase #6 $158.35   5.123%   $166.4623   $166.46   $166.45   $166.50  
  Purchase #7 $9.37   4.249%   $9.7681   $9.77   $9.75   $9.80  
  7 purchase totals = $989.49       $1,089.3046   $1,089.31   $1,089.30   $1,089.40