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 $215.64   12.435%   $242.4548   $242.45   $242.45   $242.50  
  Purchase #2 $184.55   9.178%   $201.488   $201.49   $201.50   $201.50  
  Purchase #3 $196.72   5.449%   $207.4393   $207.44   $207.45   $207.40  
  Purchase #4 $182.55   4.498%   $190.7611   $190.76   $190.75   $190.80  
  Purchase #5 $167.47   3.616%   $173.5257   $173.53   $173.55   $173.50  
  Purchase #6 $138.31   12.540%   $155.6541   $155.65   $155.65   $155.70  
  Purchase #7 $206.38   5.672%   $218.0859   $218.09   $218.10   $218.10  
  7 purchase totals = $1,291.62       $1,389.4089   $1,389.41   $1,389.45   $1,389.50