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 $79.80   13.534%   $90.6001   $90.60   $90.60   $90.60  
  Purchase #2 $163.28   11.380%   $181.8613   $181.86   $181.85   $181.90  
  Purchase #3 $209.61   5.746%   $221.6542   $221.65   $221.65   $221.70  
  Purchase #4 $206.28   13.373%   $233.8658   $233.87   $233.85   $233.90  
  Purchase #5 $105.25   8.436%   $114.1289   $114.13   $114.15   $114.10  
  Purchase #6 $57.71   7.433%   $61.9996   $62.00   $62.00   $62.00  
  Purchase #7 $109.63   11.829%   $122.5981   $122.60   $122.60   $122.60  
  7 purchase totals = $931.56       $1,026.7080   $1,026.71   $1,026.70   $1,026.80