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 $246.90   12.308%   $277.2885   $277.29   $277.30   $277.30  
  Purchase #2 $222.93   3.623%   $231.0068   $231.01   $231.00   $231.00  
  Purchase #3 $152.43   9.604%   $167.0694   $167.07   $167.05   $167.10  
  Purchase #4 $49.90   7.587%   $53.6859   $53.69   $53.70   $53.70  
  Purchase #5 $151.46   8.697%   $164.6325   $164.63   $164.65   $164.60  
  Purchase #6 $110.42   8.628%   $119.947   $119.95   $119.95   $119.90  
  Purchase #7 $231.49   10.282%   $255.2918   $255.29   $255.30   $255.30  
  7 purchase totals = $1,165.53       $1,268.9219   $1,268.93   $1,268.95   $1,268.90