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 $28.51   5.345%   $30.0339   $30.03   $30.05   $30.00  
  Purchase #2 $144.51   10.854%   $160.1951   $160.20   $160.20   $160.20  
  Purchase #3 $143.85   3.456%   $148.8215   $148.82   $148.80   $148.80  
  Purchase #4 $139.53   13.696%   $158.64   $158.64   $158.65   $158.60  
  Purchase #5 $232.78   11.227%   $258.9142   $258.91   $258.90   $258.90  
  Purchase #6 $49.98   7.543%   $53.75   $53.75   $53.75   $53.80  
  Purchase #7 $92.78   6.408%   $98.7253   $98.73   $98.75   $98.70  
  7 purchase totals = $831.94       $909.0800   $909.08   $909.10   $909.00