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 $228.55   13.139%   $258.5792   $258.58   $258.60   $258.60  
  Purchase #2 $240.90   3.502%   $249.3363   $249.34   $249.35   $249.30  
  Purchase #3 $127.99   7.550%   $137.6532   $137.65   $137.65   $137.70  
  Purchase #4 $189.39   8.481%   $205.4522   $205.45   $205.45   $205.50  
  Purchase #5 $217.91   12.461%   $245.0638   $245.06   $245.05   $245.10  
  Purchase #6 $44.36   12.655%   $49.9738   $49.97   $49.95   $50.00  
  Purchase #7 $197.43   12.296%   $221.706   $221.71   $221.70   $221.70  
  7 purchase totals = $1,246.53       $1,367.7645   $1,367.76   $1,367.75   $1,367.90