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 $129.99   4.537%   $135.8876   $135.89   $135.90   $135.90  
  Purchase #2 $243.36   5.833%   $257.5552   $257.56   $257.55   $257.60  
  Purchase #3 $244.49   4.654%   $255.8686   $255.87   $255.85   $255.90  
  Purchase #4 $176.66   9.457%   $193.3667   $193.37   $193.35   $193.40  
  Purchase #5 $35.34   3.223%   $36.479   $36.48   $36.50   $36.50  
  Purchase #6 $213.29   4.138%   $222.1159   $222.12   $222.10   $222.10  
  Purchase #7 $166.41   13.720%   $189.2415   $189.24   $189.25   $189.20  
  7 purchase totals = $1,209.54       $1,290.5145   $1,290.53   $1,290.50   $1,290.60