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 $7.40   5.600%   $7.8144   $7.81   $7.80   $7.80  
  Purchase #2 $129.64   7.556%   $139.4356   $139.44   $139.45   $139.40  
  Purchase #3 $139.71   9.525%   $153.0174   $153.02   $153.00   $153.00  
  Purchase #4 $243.83   9.735%   $267.5669   $267.57   $267.55   $267.60  
  Purchase #5 $168.80   5.689%   $178.403   $178.40   $178.40   $178.40  
  Purchase #6 $67.77   5.754%   $71.6695   $71.67   $71.65   $71.70  
  Purchase #7 $158.65   4.452%   $165.7131   $165.71   $165.70   $165.70  
  7 purchase totals = $915.80       $983.6199   $983.62   $983.55   $983.60