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 $248.25   12.693%   $279.7604   $279.76   $279.75   $279.80  
  Purchase #2 $198.48   5.221%   $208.8426   $208.84   $208.85   $208.80  
  Purchase #3 $122.97   5.488%   $129.7186   $129.72   $129.70   $129.70  
  Purchase #4 $152.82   12.221%   $171.4961   $171.50   $171.50   $171.50  
  Purchase #5 $118.38   4.664%   $123.9012   $123.90   $123.90   $123.90  
  Purchase #6 $247.71   3.341%   $255.986   $255.99   $256.00   $256.00  
  Purchase #7 $75.48   7.353%   $81.03   $81.03   $81.05   $81.00  
  7 purchase totals = $1,164.09       $1,250.7349   $1,250.74   $1,250.75   $1,250.70