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 $242.27   3.439%   $250.6017   $250.60   $250.60   $250.60  
  Purchase #2 $85.47   12.354%   $96.029   $96.03   $96.05   $96.00  
  Purchase #3 $26.94   4.422%   $28.1313   $28.13   $28.15   $28.10  
  Purchase #4 $134.68   11.216%   $149.7857   $149.79   $149.80   $149.80  
  Purchase #5 $169.54   10.834%   $187.908   $187.91   $187.90   $187.90  
  Purchase #6 $229.74   11.773%   $256.7873   $256.79   $256.80   $256.80  
  Purchase #7 $23.83   13.195%   $26.9744   $26.97   $26.95   $27.00  
  7 purchase totals = $912.47       $996.2174   $996.22   $996.25   $996.20