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 $28.53   7.644%   $30.7108   $30.71   $30.70   $30.70  
  Purchase #2 $81.82   13.634%   $92.9753   $92.98   $93.00   $93.00  
  Purchase #3 $13.28   7.604%   $14.2898   $14.29   $14.30   $14.30  
  Purchase #4 $8.44   5.380%   $8.8941   $8.89   $8.90   $8.90  
  Purchase #5 $191.84   10.264%   $211.5305   $211.53   $211.55   $211.50  
  Purchase #6 $244.94   13.778%   $278.6878   $278.69   $278.70   $278.70  
  Purchase #7 $43.74   3.430%   $45.2403   $45.24   $45.25   $45.20  
  7 purchase totals = $612.59       $682.3286   $682.33   $682.40   $682.30