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 $119.78   11.805%   $133.92   $133.92   $133.90   $133.90  
  Purchase #2 $181.74   10.162%   $200.2084   $200.21   $200.20   $200.20  
  Purchase #3 $248.82   9.726%   $273.0202   $273.02   $273.00   $273.00  
  Purchase #4 $155.34   5.169%   $163.3695   $163.37   $163.35   $163.40  
  Purchase #5 $67.31   13.248%   $76.2272   $76.23   $76.25   $76.20  
  Purchase #6 $96.49   9.221%   $105.3873   $105.39   $105.40   $105.40  
  Purchase #7 $77.96   10.586%   $86.2128   $86.21   $86.20   $86.20  
  7 purchase totals = $947.44       $1,038.3454   $1,038.35   $1,038.30   $1,038.30