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 $103.94   12.784%   $117.2277   $117.23   $117.25   $117.20  
  Purchase #2 $159.63   9.851%   $175.3552   $175.36   $175.35   $175.40  
  Purchase #3 $242.78   11.826%   $271.4912   $271.49   $271.50   $271.50  
  Purchase #4 $179.37   11.251%   $199.5509   $199.55   $199.55   $199.60  
  Purchase #5 $136.79   6.580%   $145.7908   $145.79   $145.80   $145.80  
  Purchase #6 $76.54   4.531%   $80.008   $80.01   $80.00   $80.00  
  Purchase #7 $79.76   8.151%   $86.2612   $86.26   $86.25   $86.30  
  7 purchase totals = $978.81       $1,075.6850   $1,075.69   $1,075.70   $1,075.80