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 $26.64   8.442%   $28.8889   $28.89   $28.90   $28.90  
  Purchase #2 $212.84   7.800%   $229.4415   $229.44   $229.45   $229.40  
  Purchase #3 $7.52   12.403%   $8.4527   $8.45   $8.45   $8.50  
  Purchase #4 $74.53   3.296%   $76.9865   $76.99   $77.00   $77.00  
  Purchase #5 $66.91   5.625%   $70.6737   $70.67   $70.65   $70.70  
  Purchase #6 $246.46   3.372%   $254.7706   $254.77   $254.75   $254.80  
  Purchase #7 $175.40   13.407%   $198.9159   $198.92   $198.90   $198.90  
  7 purchase totals = $810.30       $868.1298   $868.13   $868.10   $868.20