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 $19.72   6.377%   $20.9775   $20.98   $21.00   $21.00  
  Purchase #2 $70.67   7.310%   $75.836   $75.84   $75.85   $75.80  
  Purchase #3 $130.93   6.304%   $139.1838   $139.18   $139.20   $139.20  
  Purchase #4 $70.79   5.343%   $74.5723   $74.57   $74.55   $74.60  
  Purchase #5 $174.30   9.292%   $190.496   $190.50   $190.50   $190.50  
  Purchase #6 $112.49   5.833%   $119.0515   $119.05   $119.05   $119.10  
  Purchase #7 $207.68   6.462%   $221.1003   $221.10   $221.10   $221.10  
  7 purchase totals = $786.58       $841.2174   $841.22   $841.25   $841.30