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 $249.85   11.304%   $278.093   $278.09   $278.10   $278.10  
  Purchase #2 $55.95   9.259%   $61.1304   $61.13   $61.15   $61.10  
  Purchase #3 $190.97   6.243%   $202.8923   $202.89   $202.90   $202.90  
  Purchase #4 $219.76   4.686%   $230.058   $230.06   $230.05   $230.10  
  Purchase #5 $14.58   4.398%   $15.2212   $15.22   $15.20   $15.20  
  Purchase #6 $131.66   3.682%   $136.5077   $136.51   $136.50   $136.50  
  Purchase #7 $63.32   11.398%   $70.5372   $70.54   $70.55   $70.50  
  7 purchase totals = $926.09       $994.4398   $994.44   $994.45   $994.40