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 $83.45   12.396%   $93.7945   $93.79   $93.80   $93.80  
  Purchase #2 $249.98   5.151%   $262.8565   $262.86   $262.85   $262.90  
  Purchase #3 $174.62   3.312%   $180.4034   $180.40   $180.40   $180.40  
  Purchase #4 $28.87   9.284%   $31.5503   $31.55   $31.55   $31.60  
  Purchase #5 $159.99   11.328%   $178.1137   $178.11   $178.10   $178.10  
  Purchase #6 $59.94   9.507%   $65.6385   $65.64   $65.65   $65.60  
  Purchase #7 $190.30   12.698%   $214.4643   $214.46   $214.45   $214.50  
  7 purchase totals = $947.15       $1,026.8212   $1,026.81   $1,026.80   $1,026.90