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 $160.39   3.725%   $166.3645   $166.36   $166.35   $166.40  
  Purchase #2 $72.42   10.699%   $80.1682   $80.17   $80.15   $80.20  
  Purchase #3 $114.73   10.764%   $127.0795   $127.08   $127.10   $127.10  
  Purchase #4 $18.46   8.378%   $20.0066   $20.01   $20.00   $20.00  
  Purchase #5 $8.49   4.610%   $8.8814   $8.88   $8.90   $8.90  
  Purchase #6 $124.48   8.542%   $135.1131   $135.11   $135.10   $135.10  
  Purchase #7 $19.63   3.259%   $20.2697   $20.27   $20.25   $20.30  
  7 purchase totals = $518.60       $557.8830   $557.88   $557.85   $558.00