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 $56.58   6.581%   $60.3035   $60.30   $60.30   $60.30  
  Purchase #2 $163.96   11.643%   $183.0499   $183.05   $183.05   $183.00  
  Purchase #3 $196.34   9.448%   $214.8902   $214.89   $214.90   $214.90  
  Purchase #4 $34.88   5.412%   $36.7677   $36.77   $36.75   $36.80  
  Purchase #5 $68.39   12.631%   $77.0283   $77.03   $77.05   $77.00  
  Purchase #6 $26.32   9.331%   $28.7759   $28.78   $28.80   $28.80  
  Purchase #7 $159.56   13.693%   $181.4086   $181.41   $181.40   $181.40  
  7 purchase totals = $706.03       $782.2241   $782.23   $782.25   $782.20