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 $235.93   13.510%   $267.8041   $267.80   $267.80   $267.80  
  Purchase #2 $8.83   7.320%   $9.4764   $9.48   $9.50   $9.50  
  Purchase #3 $19.53   10.542%   $21.5889   $21.59   $21.60   $21.60  
  Purchase #4 $222.65   8.433%   $241.4261   $241.43   $241.45   $241.40  
  Purchase #5 $104.83   4.752%   $109.8115   $109.81   $109.80   $109.80  
  Purchase #6 $17.64   10.773%   $19.5404   $19.54   $19.55   $19.50  
  Purchase #7 $165.53   11.772%   $185.0162   $185.02   $185.00   $185.00  
  7 purchase totals = $774.94       $854.6636   $854.67   $854.70   $854.60