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 $19.95   6.634%   $21.2735   $21.27   $21.25   $21.30  
  Purchase #2 $186.70   9.332%   $204.1228   $204.12   $204.10   $204.10  
  Purchase #3 $72.41   4.732%   $75.8364   $75.84   $75.85   $75.80  
  Purchase #4 $186.41   3.512%   $192.9567   $192.96   $192.95   $193.00  
  Purchase #5 $188.55   12.109%   $211.3815   $211.38   $211.40   $211.40  
  Purchase #6 $20.88   13.373%   $23.6723   $23.67   $23.65   $23.70  
  Purchase #7 $133.27   7.246%   $142.9267   $142.93   $142.95   $142.90  
  7 purchase totals = $808.17       $872.1699   $872.17   $872.15   $872.20