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 $100.55   12.162%   $112.7789   $112.78   $112.80   $112.80  
  Purchase #2 $234.99   6.440%   $250.1234   $250.12   $250.10   $250.10  
  Purchase #3 $179.25   10.520%   $198.1071   $198.11   $198.10   $198.10  
  Purchase #4 $20.78   10.657%   $22.9945   $22.99   $23.00   $23.00  
  Purchase #5 $140.87   11.780%   $157.4645   $157.46   $157.45   $157.50  
  Purchase #6 $84.60   3.778%   $87.7962   $87.80   $87.80   $87.80  
  Purchase #7 $23.39   12.756%   $26.3736   $26.37   $26.35   $26.40  
  7 purchase totals = $784.43       $855.6382   $855.63   $855.60   $855.70