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 $223.53   11.374%   $248.9543   $248.95   $248.95   $249.00  
  Purchase #2 $178.57   12.587%   $201.0466   $201.05   $201.05   $201.00  
  Purchase #3 $244.73   12.830%   $276.1289   $276.13   $276.15   $276.10  
  Purchase #4 $43.30   4.182%   $45.1108   $45.11   $45.10   $45.10  
  Purchase #5 $67.76   9.661%   $74.3063   $74.31   $74.30   $74.30  
  Purchase #6 $147.44   6.768%   $157.4187   $157.42   $157.40   $157.40  
  Purchase #7 $249.30   11.520%   $278.0194   $278.02   $278.00   $278.00  
  7 purchase totals = $1,154.63       $1,280.9850   $1,280.99   $1,280.95   $1,280.90