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 $36.78   8.742%   $39.9953   $40.00   $40.00   $40.00  
  Purchase #2 $173.88   10.430%   $192.0157   $192.02   $192.00   $192.00  
  Purchase #3 $10.89   11.294%   $12.1199   $12.12   $12.10   $12.10  
  Purchase #4 $98.98   10.717%   $109.5877   $109.59   $109.60   $109.60  
  Purchase #5 $250.80   10.717%   $277.6782   $277.68   $277.70   $277.70  
  Purchase #6 $57.28   4.756%   $60.0042   $60.00   $60.00   $60.00  
  Purchase #7 $35.49   5.420%   $37.4136   $37.41   $37.40   $37.40  
  7 purchase totals = $664.10       $728.8146   $728.82   $728.80   $728.80