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 $80.49   10.754%   $89.1459   $89.15   $89.15   $89.10  
  Purchase #2 $29.55   4.362%   $30.839   $30.84   $30.85   $30.80  
  Purchase #3 $201.82   9.630%   $221.2553   $221.26   $221.25   $221.30  
  Purchase #4 $133.43   8.249%   $144.4366   $144.44   $144.45   $144.40  
  Purchase #5 $201.94   8.222%   $218.5435   $218.54   $218.55   $218.50  
  Purchase #6 $237.50   7.711%   $255.8136   $255.81   $255.80   $255.80  
  Purchase #7 $130.67   13.288%   $148.0334   $148.03   $148.05   $148.00  
  7 purchase totals = $1,015.40       $1,108.0673   $1,108.07   $1,108.10   $1,107.90