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 $180.87   13.761%   $205.7595   $205.76   $205.75   $205.80  
  Purchase #2 $41.87   11.143%   $46.5356   $46.54   $46.55   $46.50  
  Purchase #3 $52.81   10.269%   $58.2331   $58.23   $58.25   $58.20  
  Purchase #4 $91.37   7.648%   $98.358   $98.36   $98.35   $98.40  
  Purchase #5 $159.44   12.416%   $179.2361   $179.24   $179.25   $179.20  
  Purchase #6 $244.83   4.472%   $255.7788   $255.78   $255.80   $255.80  
  Purchase #7 $132.28   10.835%   $146.6125   $146.61   $146.60   $146.60  
  7 purchase totals = $903.47       $990.5136   $990.52   $990.55   $990.50