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 $56.47   8.772%   $61.4235   $61.42   $61.40   $61.40  
  Purchase #2 $43.38   13.669%   $49.3096   $49.31   $49.30   $49.30  
  Purchase #3 $153.95   12.606%   $173.3569   $173.36   $173.35   $173.40  
  Purchase #4 $250.94   7.614%   $270.0466   $270.05   $270.05   $270.00  
  Purchase #5 $242.56   10.498%   $268.0239   $268.02   $268.00   $268.00  
  Purchase #6 $235.82   6.218%   $250.4833   $250.48   $250.50   $250.50  
  Purchase #7 $195.88   10.641%   $216.7236   $216.72   $216.70   $216.70  
  7 purchase totals = $1,179.00       $1,289.3674   $1,289.36   $1,289.30   $1,289.30