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 $13.89   13.333%   $15.742   $15.74   $15.75   $15.70  
  Purchase #2 $128.35   4.198%   $133.7381   $133.74   $133.75   $133.70  
  Purchase #3 $207.48   3.748%   $215.2564   $215.26   $215.25   $215.30  
  Purchase #4 $145.52   8.141%   $157.3668   $157.37   $157.35   $157.40  
  Purchase #5 $18.80   5.798%   $19.89   $19.89   $19.90   $19.90  
  Purchase #6 $12.52   13.559%   $14.2176   $14.22   $14.20   $14.20  
  Purchase #7 $71.84   8.429%   $77.8954   $77.90   $77.90   $77.90  
  7 purchase totals = $598.40       $634.1063   $634.12   $634.10   $634.10