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 $17.34   3.822%   $18.0027   $18.00   $18.00   $18.00  
  Purchase #2 $6.56   12.369%   $7.3714   $7.37   $7.35   $7.40  
  Purchase #3 $128.67   8.729%   $139.9016   $139.90   $139.90   $139.90  
  Purchase #4 $231.96   9.653%   $254.3511   $254.35   $254.35   $254.40  
  Purchase #5 $248.71   12.212%   $279.0825   $279.08   $279.10   $279.10  
  Purchase #6 $64.61   13.417%   $73.2787   $73.28   $73.30   $73.30  
  Purchase #7 $189.37   5.117%   $199.0601   $199.06   $199.05   $199.10  
  7 purchase totals = $887.22       $971.0481   $971.04   $971.05   $971.20