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 $241.69   13.191%   $273.5713   $273.57   $273.55   $273.60  
  Purchase #2 $218.96   8.730%   $238.0752   $238.08   $238.10   $238.10  
  Purchase #3 $45.99   12.671%   $51.8174   $51.82   $51.80   $51.80  
  Purchase #4 $213.43   13.211%   $241.6262   $241.63   $241.65   $241.60  
  Purchase #5 $67.50   12.157%   $75.706   $75.71   $75.70   $75.70  
  Purchase #6 $108.68   9.772%   $119.3002   $119.30   $119.30   $119.30  
  Purchase #7 $125.53   8.191%   $135.8122   $135.81   $135.80   $135.80  
  7 purchase totals = $1,021.78       $1,135.9085   $1,135.92   $1,135.90   $1,135.90