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 $18.69   11.644%   $20.8663   $20.87   $20.85   $20.90  
  Purchase #2 $106.64   10.392%   $117.722   $117.72   $117.70   $117.70  
  Purchase #3 $32.69   9.822%   $35.9008   $35.90   $35.90   $35.90  
  Purchase #4 $221.50   8.407%   $240.1215   $240.12   $240.10   $240.10  
  Purchase #5 $126.63   12.614%   $142.6031   $142.60   $142.60   $142.60  
  Purchase #6 $171.36   10.409%   $189.1969   $189.20   $189.20   $189.20  
  Purchase #7 $90.70   11.842%   $101.4407   $101.44   $101.45   $101.40  
  7 purchase totals = $768.21       $847.8513   $847.85   $847.80   $847.80