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 $249.36   8.829%   $271.376   $271.38   $271.40   $271.40  
  Purchase #2 $249.99   6.701%   $266.7418   $266.74   $266.75   $266.70  
  Purchase #3 $132.88   9.504%   $145.5089   $145.51   $145.50   $145.50  
  Purchase #4 $92.73   8.471%   $100.5852   $100.59   $100.60   $100.60  
  Purchase #5 $52.68   7.355%   $56.5546   $56.55   $56.55   $56.60  
  Purchase #6 $194.69   7.158%   $208.6259   $208.63   $208.65   $208.60  
  Purchase #7 $192.48   11.348%   $214.3226   $214.32   $214.30   $214.30  
  7 purchase totals = $1,164.81       $1,263.7150   $1,263.72   $1,263.75   $1,263.70