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 $18.25   9.865%   $20.0504   $20.05   $20.05   $20.10  
  Purchase #2 $13.75   9.472%   $15.0524   $15.05   $15.05   $15.10  
  Purchase #3 $249.73   13.663%   $283.8506   $283.85   $283.85   $283.90  
  Purchase #4 $222.68   3.118%   $229.6232   $229.62   $229.60   $229.60  
  Purchase #5 $107.31   7.776%   $115.6544   $115.65   $115.65   $115.70  
  Purchase #6 $171.50   12.824%   $193.4932   $193.49   $193.50   $193.50  
  Purchase #7 $179.25   6.855%   $191.5376   $191.54   $191.55   $191.50  
  7 purchase totals = $962.47       $1,049.2618   $1,049.25   $1,049.25   $1,049.40