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 $8.35   6.738%   $8.9126   $8.91   $8.90   $8.90  
  Purchase #2 $71.85   10.582%   $79.4532   $79.45   $79.45   $79.50  
  Purchase #3 $143.37   9.845%   $157.4848   $157.48   $157.50   $157.50  
  Purchase #4 $28.73   7.321%   $30.8333   $30.83   $30.85   $30.80  
  Purchase #5 $181.70   4.215%   $189.3587   $189.36   $189.35   $189.40  
  Purchase #6 $84.68   4.320%   $88.3382   $88.34   $88.35   $88.30  
  Purchase #7 $9.60   7.431%   $10.3134   $10.31   $10.30   $10.30  
  7 purchase totals = $528.28       $564.6942   $564.68   $564.70   $564.70