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 $18.67   8.729%   $20.2997   $20.30   $20.30   $20.30  
  Purchase #2 $31.40   10.650%   $34.7441   $34.74   $34.75   $34.70  
  Purchase #3 $189.36   6.824%   $202.2819   $202.28   $202.30   $202.30  
  Purchase #4 $226.76   10.343%   $250.2138   $250.21   $250.20   $250.20  
  Purchase #5 $97.42   12.302%   $109.4046   $109.40   $109.40   $109.40  
  Purchase #6 $172.89   8.244%   $187.1431   $187.14   $187.15   $187.10  
  Purchase #7 $104.83   7.548%   $112.7426   $112.74   $112.75   $112.70  
  7 purchase totals = $841.33       $916.8298   $916.81   $916.85   $916.70