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 $92.35   6.594%   $98.4396   $98.44   $98.45   $98.40  
  Purchase #2 $5.98   4.827%   $6.2687   $6.27   $6.25   $6.30  
  Purchase #3 $57.84   10.830%   $64.1041   $64.10   $64.10   $64.10  
  Purchase #4 $94.30   11.234%   $104.8937   $104.89   $104.90   $104.90  
  Purchase #5 $111.77   7.773%   $120.4579   $120.46   $120.45   $120.50  
  Purchase #6 $222.32   5.411%   $234.3497   $234.35   $234.35   $234.30  
  Purchase #7 $35.68   11.136%   $39.6533   $39.65   $39.65   $39.70  
  7 purchase totals = $620.24       $668.1670   $668.16   $668.15   $668.20