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 $246.61   13.162%   $279.0688   $279.07   $279.05   $279.10  
  Purchase #2 $239.45   5.512%   $252.6485   $252.65   $252.65   $252.60  
  Purchase #3 $163.89   5.486%   $172.881   $172.88   $172.90   $172.90  
  Purchase #4 $52.69   6.440%   $56.0832   $56.08   $56.10   $56.10  
  Purchase #5 $220.85   4.222%   $230.1743   $230.17   $230.15   $230.20  
  Purchase #6 $10.95   11.204%   $12.1768   $12.18   $12.20   $12.20  
  Purchase #7 $247.88   4.163%   $258.1992   $258.20   $258.20   $258.20  
  7 purchase totals = $1,182.32       $1,261.2318   $1,261.23   $1,261.25   $1,261.30