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 $84.86   5.139%   $89.221   $89.22   $89.20   $89.20  
  Purchase #2 $151.40   3.657%   $156.9367   $156.94   $156.95   $156.90  
  Purchase #3 $9.39   9.330%   $10.2661   $10.27   $10.25   $10.30  
  Purchase #4 $248.59   10.330%   $274.2693   $274.27   $274.25   $274.30  
  Purchase #5 $92.78   13.160%   $104.9898   $104.99   $105.00   $105.00  
  Purchase #6 $241.26   11.669%   $269.4126   $269.41   $269.40   $269.40  
  Purchase #7 $104.71   11.407%   $116.6543   $116.65   $116.65   $116.70  
  7 purchase totals = $932.99       $1,021.7498   $1,021.75   $1,021.70   $1,021.80