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 $240.42   10.185%   $264.9068   $264.91   $264.90   $264.90  
  Purchase #2 $109.81   8.867%   $119.5469   $119.55   $119.55   $119.50  
  Purchase #3 $124.43   8.205%   $134.6395   $134.64   $134.65   $134.60  
  Purchase #4 $242.49   4.449%   $253.2784   $253.28   $253.30   $253.30  
  Purchase #5 $31.39   10.760%   $34.7676   $34.77   $34.75   $34.80  
  Purchase #6 $32.81   4.574%   $34.3107   $34.31   $34.30   $34.30  
  Purchase #7 $8.37   3.620%   $8.673   $8.67   $8.65   $8.70  
  7 purchase totals = $789.72       $850.1229   $850.13   $850.10   $850.10