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   13.279%   $24.6948   $24.69   $24.70   $24.70  
  Purchase #2 $214.71   12.409%   $241.3534   $241.35   $241.35   $241.40  
  Purchase #3 $158.79   13.572%   $180.341   $180.34   $180.35   $180.30  
  Purchase #4 $98.91   10.496%   $109.2916   $109.29   $109.30   $109.30  
  Purchase #5 $63.32   5.754%   $66.9634   $66.96   $66.95   $67.00  
  Purchase #6 $117.33   8.243%   $127.0015   $127.00   $127.00   $127.00  
  Purchase #7 $82.36   8.486%   $89.3491   $89.35   $89.35   $89.30  
  7 purchase totals = $757.22       $838.9948   $838.98   $839.00   $839.00