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 $211.80   13.298%   $239.9652   $239.97   $239.95   $240.00  
  Purchase #2 $165.71   9.259%   $181.0531   $181.05   $181.05   $181.10  
  Purchase #3 $77.36   11.440%   $86.21   $86.21   $86.20   $86.20  
  Purchase #4 $106.78   11.382%   $118.9337   $118.93   $118.95   $118.90  
  Purchase #5 $234.43   7.607%   $252.2631   $252.26   $252.25   $252.30  
  Purchase #6 $201.39   13.657%   $228.8938   $228.89   $228.90   $228.90  
  Purchase #7 $135.54   7.588%   $145.8248   $145.82   $145.80   $145.80  
  7 purchase totals = $1,133.01       $1,253.1437   $1,253.13   $1,253.10   $1,253.20