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 $100.92   6.414%   $107.393   $107.39   $107.40   $107.40  
  Purchase #2 $73.70   9.645%   $80.8084   $80.81   $80.80   $80.80  
  Purchase #3 $215.61   12.602%   $242.7812   $242.78   $242.80   $242.80  
  Purchase #4 $165.63   10.606%   $183.1967   $183.20   $183.20   $183.20  
  Purchase #5 $214.33   8.259%   $232.0315   $232.03   $232.05   $232.00  
  Purchase #6 $86.87   4.499%   $90.7783   $90.78   $90.80   $90.80  
  Purchase #7 $47.42   8.230%   $51.3227   $51.32   $51.30   $51.30  
  7 purchase totals = $904.48       $988.3118   $988.31   $988.35   $988.30