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 $21.80   11.320%   $24.2678   $24.27   $24.25   $24.30  
  Purchase #2 $93.28   7.771%   $100.5288   $100.53   $100.55   $100.50  
  Purchase #3 $92.58   12.536%   $104.1858   $104.19   $104.20   $104.20  
  Purchase #4 $32.40   10.664%   $35.8551   $35.86   $35.85   $35.90  
  Purchase #5 $136.46   6.254%   $144.9942   $144.99   $145.00   $145.00  
  Purchase #6 $25.99   6.851%   $27.7706   $27.77   $27.75   $27.80  
  Purchase #7 $248.91   8.662%   $270.4706   $270.47   $270.45   $270.50  
  7 purchase totals = $651.42       $708.0729   $708.08   $708.05   $708.20