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 $223.39   11.687%   $249.4976   $249.50   $249.50   $249.50  
  Purchase #2 $247.93   9.165%   $270.6528   $270.65   $270.65   $270.70  
  Purchase #3 $18.50   8.803%   $20.1286   $20.13   $20.15   $20.10  
  Purchase #4 $202.68   3.310%   $209.3887   $209.39   $209.40   $209.40  
  Purchase #5 $30.81   6.377%   $32.7748   $32.77   $32.75   $32.80  
  Purchase #6 $186.64   7.770%   $201.1419   $201.14   $201.15   $201.10  
  Purchase #7 $234.72   6.500%   $249.9768   $249.98   $250.00   $250.00  
  7 purchase totals = $1,144.67       $1,233.5612   $1,233.56   $1,233.60   $1,233.60