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 $42.33   6.378%   $45.0298   $45.03   $45.05   $45.00  
  Purchase #2 $155.42   6.651%   $165.757   $165.76   $165.75   $165.80  
  Purchase #3 $142.50   13.544%   $161.8002   $161.80   $161.80   $161.80  
  Purchase #4 $124.51   4.747%   $130.4205   $130.42   $130.40   $130.40  
  Purchase #5 $52.42   6.482%   $55.8179   $55.82   $55.80   $55.80  
  Purchase #6 $232.28   7.176%   $248.9484   $248.95   $248.95   $248.90  
  Purchase #7 $7.42   5.542%   $7.8312   $7.83   $7.85   $7.80  
  7 purchase totals = $756.88       $815.6050   $815.61   $815.60   $815.50