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 $29.34   5.608%   $30.9854   $30.99   $31.00   $31.00  
  Purchase #2 $73.88   11.124%   $82.0984   $82.10   $82.10   $82.10  
  Purchase #3 $232.90   12.121%   $261.1298   $261.13   $261.15   $261.10  
  Purchase #4 $34.82   10.697%   $38.5447   $38.54   $38.55   $38.50  
  Purchase #5 $34.58   6.569%   $36.8516   $36.85   $36.85   $36.90  
  Purchase #6 $110.45   13.801%   $125.6932   $125.69   $125.70   $125.70  
  Purchase #7 $98.98   13.145%   $111.9909   $111.99   $112.00   $112.00  
  7 purchase totals = $614.95       $687.2940   $687.29   $687.35   $687.30