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 $249.75   10.859%   $276.8704   $276.87   $276.85   $276.90  
  Purchase #2 $135.94   4.779%   $142.4366   $142.44   $142.45   $142.40  
  Purchase #3 $204.72   8.305%   $221.722   $221.72   $221.70   $221.70  
  Purchase #4 $115.39   4.699%   $120.8122   $120.81   $120.80   $120.80  
  Purchase #5 $167.33   5.672%   $176.821   $176.82   $176.80   $176.80  
  Purchase #6 $167.89   10.487%   $185.4966   $185.50   $185.50   $185.50  
  Purchase #7 $171.95   4.729%   $180.0815   $180.08   $180.10   $180.10  
  7 purchase totals = $1,212.97       $1,304.2403   $1,304.24   $1,304.20   $1,304.20