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 $191.61   7.489%   $205.9597   $205.96   $205.95   $206.00  
  Purchase #2 $102.32   11.741%   $114.3334   $114.33   $114.35   $114.30  
  Purchase #3 $115.58   10.131%   $127.2894   $127.29   $127.30   $127.30  
  Purchase #4 $229.59   12.326%   $257.8893   $257.89   $257.90   $257.90  
  Purchase #5 $157.27   3.642%   $162.9978   $163.00   $163.00   $163.00  
  Purchase #6 $24.36   9.819%   $26.7519   $26.75   $26.75   $26.80  
  Purchase #7 $89.65   6.831%   $95.774   $95.77   $95.75   $95.80  
  7 purchase totals = $910.38       $990.9955   $990.99   $991.00   $991.10