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 $37.82   5.745%   $39.9928   $39.99   $40.00   $40.00  
  Purchase #2 $109.91   11.808%   $122.8882   $122.89   $122.90   $122.90  
  Purchase #3 $197.39   4.128%   $205.5383   $205.54   $205.55   $205.50  
  Purchase #4 $114.95   13.573%   $130.5522   $130.55   $130.55   $130.60  
  Purchase #5 $209.74   12.218%   $235.366   $235.37   $235.35   $235.40  
  Purchase #6 $151.76   10.750%   $168.0742   $168.07   $168.05   $168.10  
  Purchase #7 $178.53   4.257%   $186.13   $186.13   $186.15   $186.10  
  7 purchase totals = $1,000.10       $1,088.5417   $1,088.54   $1,088.55   $1,088.60