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 $19.84   8.574%   $21.5411   $21.54   $21.55   $21.50  
  Purchase #2 $140.30   10.694%   $155.3037   $155.30   $155.30   $155.30  
  Purchase #3 $216.90   3.163%   $223.7605   $223.76   $223.75   $223.80  
  Purchase #4 $20.31   13.870%   $23.127   $23.13   $23.15   $23.10  
  Purchase #5 $239.99   7.152%   $257.1541   $257.15   $257.15   $257.20  
  Purchase #6 $39.72   8.448%   $43.0755   $43.08   $43.10   $43.10  
  Purchase #7 $8.99   4.504%   $9.3949   $9.39   $9.40   $9.40  
  7 purchase totals = $686.05       $733.3568   $733.35   $733.40   $733.40