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 $19.61   7.378%   $21.0568   $21.06   $21.05   $21.10  
  Purchase #2 $7.28   7.556%   $7.8301   $7.83   $7.85   $7.80  
  Purchase #3 $90.68   12.544%   $102.0549   $102.05   $102.05   $102.10  
  Purchase #4 $152.34   3.727%   $158.0177   $158.02   $158.00   $158.00  
  Purchase #5 $225.88   10.609%   $249.8436   $249.84   $249.85   $249.80  
  Purchase #6 $76.60   3.837%   $79.5391   $79.54   $79.55   $79.50  
  Purchase #7 $94.47   10.168%   $104.0757   $104.08   $104.10   $104.10  
  7 purchase totals = $666.86       $722.4179   $722.42   $722.45   $722.40