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 $19.48   9.333%   $21.2981   $21.30   $21.30   $21.30  
  Purchase #2 $219.72   6.645%   $234.3204   $234.32   $234.30   $234.30  
  Purchase #3 $5.92   9.530%   $6.4842   $6.48   $6.50   $6.50  
  Purchase #4 $44.35   7.107%   $47.502   $47.50   $47.50   $47.50  
  Purchase #5 $149.90   8.552%   $162.7194   $162.72   $162.70   $162.70  
  Purchase #6 $17.64   12.632%   $19.8683   $19.87   $19.85   $19.90  
  Purchase #7 $71.32   13.205%   $80.7378   $80.74   $80.75   $80.70  
  7 purchase totals = $528.33       $572.9302   $572.93   $572.90   $572.90