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 $18.87   11.116%   $20.9676   $20.97   $20.95   $21.00  
  Purchase #2 $70.70   13.275%   $80.0854   $80.09   $80.10   $80.10  
  Purchase #3 $221.68   8.474%   $240.4652   $240.47   $240.45   $240.50  
  Purchase #4 $103.38   7.468%   $111.1004   $111.10   $111.10   $111.10  
  Purchase #5 $88.67   12.254%   $99.5356   $99.54   $99.55   $99.50  
  Purchase #6 $101.71   10.767%   $112.6611   $112.66   $112.65   $112.70  
  Purchase #7 $23.63   6.729%   $25.2201   $25.22   $25.20   $25.20  
  7 purchase totals = $628.64       $690.0354   $690.05   $690.00   $690.10