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 $127.74   6.808%   $136.4365   $136.44   $136.45   $136.40  
  Purchase #2 $205.62   12.344%   $231.0017   $231.00   $231.00   $231.00  
  Purchase #3 $167.69   3.385%   $173.3663   $173.37   $173.35   $173.40  
  Purchase #4 $231.59   13.577%   $263.033   $263.03   $263.05   $263.00  
  Purchase #5 $226.36   3.590%   $234.4863   $234.49   $234.50   $234.50  
  Purchase #6 $186.99   4.543%   $195.485   $195.49   $195.50   $195.50  
  Purchase #7 $54.74   7.847%   $59.0354   $59.04   $59.05   $59.00  
  7 purchase totals = $1,200.73       $1,292.8442   $1,292.86   $1,292.90   $1,292.80