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 $243.90   12.235%   $273.7412   $273.74   $273.75   $273.70  
  Purchase #2 $87.76   10.728%   $97.1749   $97.17   $97.15   $97.20  
  Purchase #3 $73.39   3.205%   $75.7421   $75.74   $75.75   $75.70  
  Purchase #4 $173.51   9.379%   $189.7835   $189.78   $189.80   $189.80  
  Purchase #5 $96.65   9.159%   $105.5022   $105.50   $105.50   $105.50  
  Purchase #6 $53.71   9.383%   $58.7496   $58.75   $58.75   $58.70  
  Purchase #7 $117.84   5.597%   $124.4355   $124.44   $124.45   $124.40  
  7 purchase totals = $846.76       $925.1290   $925.12   $925.15   $925.00