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 $232.97   7.294%   $249.9628   $249.96   $249.95   $250.00  
  Purchase #2 $215.32   9.383%   $235.5235   $235.52   $235.50   $235.50  
  Purchase #3 $179.87   7.128%   $192.6911   $192.69   $192.70   $192.70  
  Purchase #4 $206.53   9.318%   $225.7745   $225.77   $225.75   $225.80  
  Purchase #5 $184.51   4.376%   $192.5842   $192.58   $192.60   $192.60  
  Purchase #6 $36.92   4.683%   $38.649   $38.65   $38.65   $38.60  
  Purchase #7 $173.82   5.599%   $183.5522   $183.55   $183.55   $183.60  
  7 purchase totals = $1,229.94       $1,318.7373   $1,318.72   $1,318.70   $1,318.80