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 $8.50   8.770%   $9.2455   $9.25   $9.25   $9.20  
  Purchase #2 $87.92   3.188%   $90.7229   $90.72   $90.70   $90.70  
  Purchase #3 $13.61   12.315%   $15.2861   $15.29   $15.30   $15.30  
  Purchase #4 $9.79   7.538%   $10.528   $10.53   $10.55   $10.50  
  Purchase #5 $20.54   13.167%   $23.2445   $23.24   $23.25   $23.20  
  Purchase #6 $186.64   7.395%   $200.442   $200.44   $200.45   $200.40  
  Purchase #7 $50.96   4.830%   $53.4214   $53.42   $53.40   $53.40  
  7 purchase totals = $377.96       $402.8904   $402.89   $402.90   $402.70