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 $60.53   13.139%   $68.483   $68.48   $68.50   $68.50  
  Purchase #2 $102.79   4.252%   $107.1606   $107.16   $107.15   $107.20  
  Purchase #3 $245.97   5.295%   $258.9941   $258.99   $259.00   $259.00  
  Purchase #4 $130.58   11.323%   $145.3656   $145.37   $145.35   $145.40  
  Purchase #5 $219.54   10.613%   $242.8398   $242.84   $242.85   $242.80  
  Purchase #6 $73.27   5.812%   $77.5285   $77.53   $77.55   $77.50  
  Purchase #7 $8.39   8.615%   $9.1128   $9.11   $9.10   $9.10  
  7 purchase totals = $841.07       $909.4844   $909.48   $909.50   $909.50