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 $239.79   10.104%   $264.0184   $264.02   $264.00   $264.00  
  Purchase #2 $136.93   8.278%   $148.2651   $148.27   $148.25   $148.30  
  Purchase #3 $29.69   13.784%   $33.7825   $33.78   $33.80   $33.80  
  Purchase #4 $240.43   8.259%   $260.2871   $260.29   $260.30   $260.30  
  Purchase #5 $182.30   11.203%   $202.7231   $202.72   $202.70   $202.70  
  Purchase #6 $12.74   7.755%   $13.728   $13.73   $13.75   $13.70  
  Purchase #7 $103.80   3.119%   $107.0375   $107.04   $107.05   $107.00  
  7 purchase totals = $945.68       $1,029.8417   $1,029.85   $1,029.85   $1,029.80