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 $236.82   12.366%   $266.1052   $266.11   $266.10   $266.10  
  Purchase #2 $44.31   11.871%   $49.57   $49.57   $49.55   $49.60  
  Purchase #3 $224.39   13.839%   $255.4433   $255.44   $255.45   $255.40  
  Purchase #4 $137.67   11.593%   $153.6301   $153.63   $153.65   $153.60  
  Purchase #5 $36.26   8.193%   $39.2308   $39.23   $39.25   $39.20  
  Purchase #6 $184.25   12.573%   $207.4158   $207.42   $207.40   $207.40  
  Purchase #7 $76.55   10.346%   $84.4699   $84.47   $84.45   $84.50  
  7 purchase totals = $940.25       $1,055.8651   $1,055.87   $1,055.85   $1,055.80