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 $7.84   4.171%   $8.167   $8.17   $8.15   $8.20  
  Purchase #2 $14.53   12.602%   $16.3611   $16.36   $16.35   $16.40  
  Purchase #3 $167.47   10.663%   $185.3273   $185.33   $185.35   $185.30  
  Purchase #4 $101.67   7.603%   $109.4   $109.40   $109.40   $109.40  
  Purchase #5 $200.71   11.445%   $223.6813   $223.68   $223.70   $223.70  
  Purchase #6 $73.84   10.659%   $81.7106   $81.71   $81.70   $81.70  
  Purchase #7 $12.91   5.258%   $13.5888   $13.59   $13.60   $13.60  
  7 purchase totals = $578.97       $638.2361   $638.24   $638.25   $638.30