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 $241.58   13.118%   $273.2705   $273.27   $273.25   $273.30  
  Purchase #2 $61.91   6.558%   $65.9701   $65.97   $65.95   $66.00  
  Purchase #3 $143.59   10.423%   $158.5564   $158.56   $158.55   $158.60  
  Purchase #4 $215.64   12.184%   $241.9136   $241.91   $241.90   $241.90  
  Purchase #5 $77.94   5.162%   $81.9633   $81.96   $81.95   $82.00  
  Purchase #6 $242.85   3.329%   $250.9345   $250.93   $250.95   $250.90  
  Purchase #7 $123.46   9.591%   $135.301   $135.30   $135.30   $135.30  
  7 purchase totals = $1,106.97       $1,207.9094   $1,207.90   $1,207.85   $1,208.00