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 $250.36   10.516%   $276.6879   $276.69   $276.70   $276.70  
  Purchase #2 $178.38   13.554%   $202.5576   $202.56   $202.55   $202.60  
  Purchase #3 $150.69   10.795%   $166.957   $166.96   $166.95   $167.00  
  Purchase #4 $58.26   5.754%   $61.6123   $61.61   $61.60   $61.60  
  Purchase #5 $185.94   6.788%   $198.5616   $198.56   $198.55   $198.60  
  Purchase #6 $195.57   3.243%   $201.9123   $201.91   $201.90   $201.90  
  Purchase #7 $127.80   10.221%   $140.8624   $140.86   $140.85   $140.90  
  7 purchase totals = $1,147.00       $1,249.1511   $1,249.15   $1,249.10   $1,249.30