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 $36.60   9.443%   $40.0561   $40.06   $40.05   $40.10  
  Purchase #2 $131.51   4.405%   $137.303   $137.30   $137.30   $137.30  
  Purchase #3 $244.55   8.392%   $265.0726   $265.07   $265.05   $265.10  
  Purchase #4 $7.75   5.681%   $8.1903   $8.19   $8.20   $8.20  
  Purchase #5 $213.56   3.517%   $221.0709   $221.07   $221.05   $221.10  
  Purchase #6 $13.38   10.659%   $14.8062   $14.81   $14.80   $14.80  
  Purchase #7 $198.68   3.336%   $205.308   $205.31   $205.30   $205.30  
  7 purchase totals = $846.03       $891.8071   $891.81   $891.75   $891.90