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 $102.70   5.651%   $108.5036   $108.50   $108.50   $108.50  
  Purchase #2 $233.78   12.418%   $262.8108   $262.81   $262.80   $262.80  
  Purchase #3 $90.65   11.240%   $100.8391   $100.84   $100.85   $100.80  
  Purchase #4 $88.32   7.435%   $94.8866   $94.89   $94.90   $94.90  
  Purchase #5 $8.52   10.801%   $9.4402   $9.44   $9.45   $9.40  
  Purchase #6 $21.98   3.256%   $22.6957   $22.70   $22.70   $22.70  
  Purchase #7 $37.44   3.548%   $38.7684   $38.77   $38.75   $38.80  
  7 purchase totals = $583.39       $637.9444   $637.95   $637.95   $637.90