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 $7.34   6.694%   $7.8313   $7.83   $7.85   $7.80  
  Purchase #2 $39.44   7.774%   $42.5061   $42.51   $42.50   $42.50  
  Purchase #3 $175.90   6.200%   $186.8058   $186.81   $186.80   $186.80  
  Purchase #4 $90.48   9.341%   $98.9317   $98.93   $98.95   $98.90  
  Purchase #5 $18.98   5.190%   $19.9651   $19.97   $19.95   $20.00  
  Purchase #6 $31.94   12.668%   $35.9862   $35.99   $36.00   $36.00  
  Purchase #7 $180.33   6.706%   $192.4229   $192.42   $192.40   $192.40  
  7 purchase totals = $544.41       $584.4491   $584.46   $584.45   $584.40