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 $88.87   9.412%   $97.2344   $97.23   $97.25   $97.20  
  Purchase #2 $158.68   13.326%   $179.8257   $179.83   $179.85   $179.80  
  Purchase #3 $124.68   12.769%   $140.6004   $140.60   $140.60   $140.60  
  Purchase #4 $141.33   13.492%   $160.3982   $160.40   $160.40   $160.40  
  Purchase #5 $128.50   8.541%   $139.4752   $139.48   $139.50   $139.50  
  Purchase #6 $70.31   12.414%   $79.0383   $79.04   $79.05   $79.00  
  Purchase #7 $232.90   4.167%   $242.6049   $242.60   $242.60   $242.60  
  7 purchase totals = $945.27       $1,039.1771   $1,039.18   $1,039.25   $1,039.10