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 $19.54   7.376%   $20.9813   $20.98   $21.00   $21.00  
  Purchase #2 $74.85   4.351%   $78.1067   $78.11   $78.10   $78.10  
  Purchase #3 $248.41   7.552%   $267.1699   $267.17   $267.15   $267.20  
  Purchase #4 $127.91   12.379%   $143.744   $143.74   $143.75   $143.70  
  Purchase #5 $137.37   11.259%   $152.8365   $152.84   $152.85   $152.80  
  Purchase #6 $107.27   11.327%   $119.4205   $119.42   $119.40   $119.40  
  Purchase #7 $53.85   7.836%   $58.0697   $58.07   $58.05   $58.10  
  7 purchase totals = $769.20       $840.3286   $840.33   $840.30   $840.30