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 $246.89   13.207%   $279.4968   $279.50   $279.50   $279.50  
  Purchase #2 $86.27   9.836%   $94.7555   $94.76   $94.75   $94.80  
  Purchase #3 $181.89   6.196%   $193.1599   $193.16   $193.15   $193.20  
  Purchase #4 $157.89   8.788%   $171.7654   $171.77   $171.75   $171.80  
  Purchase #5 $242.33   5.527%   $255.7236   $255.72   $255.70   $255.70  
  Purchase #6 $245.42   5.810%   $259.6789   $259.68   $259.70   $259.70  
  Purchase #7 $243.80   5.233%   $256.5581   $256.56   $256.55   $256.60  
  7 purchase totals = $1,404.49       $1,511.1382   $1,511.15   $1,511.10   $1,511.30