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 $231.53   9.816%   $254.257   $254.26   $254.25   $254.30  
  Purchase #2 $23.65   10.359%   $26.0999   $26.10   $26.10   $26.10  
  Purchase #3 $62.42   9.642%   $68.4385   $68.44   $68.45   $68.40  
  Purchase #4 $207.99   7.747%   $224.103   $224.10   $224.10   $224.10  
  Purchase #5 $143.51   5.240%   $151.0299   $151.03   $151.05   $151.00  
  Purchase #6 $15.58   13.344%   $17.659   $17.66   $17.65   $17.70  
  Purchase #7 $226.90   13.450%   $257.4181   $257.42   $257.40   $257.40  
  7 purchase totals = $911.58       $999.0054   $999.01   $999.00   $999.00