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 $20.42   4.368%   $21.3119   $21.31   $21.30   $21.30  
  Purchase #2 $154.26   11.530%   $172.0462   $172.05   $172.05   $172.00  
  Purchase #3 $170.53   10.395%   $188.2566   $188.26   $188.25   $188.30  
  Purchase #4 $34.25   13.523%   $38.8816   $38.88   $38.90   $38.90  
  Purchase #5 $17.97   10.403%   $19.8394   $19.84   $19.85   $19.80  
  Purchase #6 $29.41   12.491%   $33.0836   $33.08   $33.10   $33.10  
  Purchase #7 $42.80   8.242%   $46.3276   $46.33   $46.35   $46.30  
  7 purchase totals = $469.64       $519.7469   $519.75   $519.80   $519.70