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 $235.37   3.422%   $243.4244   $243.42   $243.40   $243.40  
  Purchase #2 $72.55   10.182%   $79.937   $79.94   $79.95   $79.90  
  Purchase #3 $135.31   6.788%   $144.4948   $144.49   $144.50   $144.50  
  Purchase #4 $15.88   7.806%   $17.1196   $17.12   $17.10   $17.10  
  Purchase #5 $226.53   8.562%   $245.9255   $245.93   $245.95   $245.90  
  Purchase #6 $73.45   8.441%   $79.6499   $79.65   $79.65   $79.60  
  Purchase #7 $234.81   6.482%   $250.0304   $250.03   $250.05   $250.00  
  7 purchase totals = $993.90       $1,060.5816   $1,060.58   $1,060.60   $1,060.40