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 $127.30   8.626%   $138.2809   $138.28   $138.30   $138.30  
  Purchase #2 $30.91   9.249%   $33.7689   $33.77   $33.75   $33.80  
  Purchase #3 $25.89   9.551%   $28.3628   $28.36   $28.35   $28.40  
  Purchase #4 $13.44   10.706%   $14.8789   $14.88   $14.90   $14.90  
  Purchase #5 $66.26   4.426%   $69.1927   $69.19   $69.20   $69.20  
  Purchase #6 $166.34   13.164%   $188.237   $188.24   $188.25   $188.20  
  Purchase #7 $150.34   3.580%   $155.7222   $155.72   $155.70   $155.70  
  7 purchase totals = $580.48       $628.4434   $628.44   $628.45   $628.50