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 $247.99   12.501%   $278.9912   $278.99   $279.00   $279.00  
  Purchase #2 $198.48   8.151%   $214.6581   $214.66   $214.65   $214.70  
  Purchase #3 $42.71   5.250%   $44.9523   $44.95   $44.95   $45.00  
  Purchase #4 $193.90   8.252%   $209.9006   $209.90   $209.90   $209.90  
  Purchase #5 $241.90   11.459%   $269.6193   $269.62   $269.60   $269.60  
  Purchase #6 $111.51   12.304%   $125.2302   $125.23   $125.25   $125.20  
  Purchase #7 $90.30   13.181%   $102.2024   $102.20   $102.20   $102.20  
  7 purchase totals = $1,126.79       $1,245.5541   $1,245.55   $1,245.55   $1,245.60