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 $36.61   7.697%   $39.4279   $39.43   $39.45   $39.40  
  Purchase #2 $142.83   4.180%   $148.8003   $148.80   $148.80   $148.80  
  Purchase #3 $87.35   9.632%   $95.7636   $95.76   $95.75   $95.80  
  Purchase #4 $16.69   7.823%   $17.9957   $18.00   $18.00   $18.00  
  Purchase #5 $189.70   8.652%   $206.1128   $206.11   $206.10   $206.10  
  Purchase #6 $114.99   4.705%   $120.4003   $120.40   $120.40   $120.40  
  Purchase #7 $9.33   4.531%   $9.7527   $9.75   $9.75   $9.80  
  7 purchase totals = $597.50       $638.2533   $638.25   $638.25   $638.30