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 $24.81   12.810%   $27.9882   $27.99   $28.00   $28.00  
  Purchase #2 $45.64   10.589%   $50.4728   $50.47   $50.45   $50.50  
  Purchase #3 $40.80   7.260%   $43.7621   $43.76   $43.75   $43.80  
  Purchase #4 $205.75   7.832%   $221.8643   $221.86   $221.85   $221.90  
  Purchase #5 $183.45   13.795%   $208.7569   $208.76   $208.75   $208.80  
  Purchase #6 $71.95   7.364%   $77.2484   $77.25   $77.25   $77.20  
  Purchase #7 $194.85   8.678%   $211.7591   $211.76   $211.75   $211.80  
  7 purchase totals = $767.25       $841.8518   $841.85   $841.80   $842.00