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 $21.85   4.754%   $22.8887   $22.89   $22.90   $22.90  
  Purchase #2 $172.80   12.148%   $193.7917   $193.79   $193.80   $193.80  
  Purchase #3 $181.95   13.862%   $207.1719   $207.17   $207.15   $207.20  
  Purchase #4 $237.99   3.276%   $245.7866   $245.79   $245.80   $245.80  
  Purchase #5 $25.75   5.785%   $27.2396   $27.24   $27.25   $27.20  
  Purchase #6 $82.79   7.863%   $89.2998   $89.30   $89.30   $89.30  
  Purchase #7 $113.63   13.469%   $128.9348   $128.93   $128.95   $128.90  
  7 purchase totals = $836.76       $915.1131   $915.11   $915.15   $915.10