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 $8.34   9.160%   $9.1039   $9.10   $9.10   $9.10  
  Purchase #2 $9.37   9.357%   $10.2468   $10.25   $10.25   $10.20  
  Purchase #3 $29.42   4.432%   $30.7239   $30.72   $30.70   $30.70  
  Purchase #4 $122.61   7.149%   $131.3754   $131.38   $131.40   $131.40  
  Purchase #5 $59.71   10.261%   $65.8368   $65.84   $65.85   $65.80  
  Purchase #6 $94.27   7.196%   $101.0537   $101.05   $101.05   $101.10  
  Purchase #7 $186.25   11.807%   $208.2405   $208.24   $208.25   $208.20  
  7 purchase totals = $509.97       $556.5810   $556.58   $556.60   $556.50