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 $96.63   7.119%   $103.5091   $103.51   $103.50   $103.50  
  Purchase #2 $56.59   6.432%   $60.2299   $60.23   $60.25   $60.20  
  Purchase #3 $201.93   3.255%   $208.5028   $208.50   $208.50   $208.50  
  Purchase #4 $61.60   3.267%   $63.6125   $63.61   $63.60   $63.60  
  Purchase #5 $67.78   10.455%   $74.8664   $74.87   $74.85   $74.90  
  Purchase #6 $191.41   7.794%   $206.3285   $206.33   $206.35   $206.30  
  Purchase #7 $194.79   4.693%   $203.9315   $203.93   $203.95   $203.90  
  7 purchase totals = $870.73       $920.9807   $920.98   $921.00   $920.90