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 $13.96   10.339%   $15.4033   $15.40   $15.40   $15.40  
  Purchase #2 $63.25   13.868%   $72.0215   $72.02   $72.00   $72.00  
  Purchase #3 $74.77   13.591%   $84.932   $84.93   $84.95   $84.90  
  Purchase #4 $147.52   4.619%   $154.3339   $154.33   $154.35   $154.30  
  Purchase #5 $95.96   9.863%   $105.4245   $105.42   $105.40   $105.40  
  Purchase #6 $27.98   7.642%   $30.1182   $30.12   $30.10   $30.10  
  Purchase #7 $219.74   3.418%   $227.2507   $227.25   $227.25   $227.30  
  7 purchase totals = $643.18       $689.4841   $689.47   $689.45   $689.40