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 $6.58   6.133%   $6.9836   $6.98   $7.00   $7.00  
  Purchase #2 $57.40   8.828%   $62.4673   $62.47   $62.45   $62.50  
  Purchase #3 $28.45   5.132%   $29.9101   $29.91   $29.90   $29.90  
  Purchase #4 $98.71   9.862%   $108.4448   $108.44   $108.45   $108.40  
  Purchase #5 $14.90   4.542%   $15.5768   $15.58   $15.60   $15.60  
  Purchase #6 $7.56   8.506%   $8.2031   $8.20   $8.20   $8.20  
  Purchase #7 $150.30   5.286%   $158.2449   $158.24   $158.25   $158.20  
  7 purchase totals = $363.90       $389.8306   $389.82   $389.85   $389.80