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 $18.61   13.243%   $21.0745   $21.07   $21.05   $21.10  
  Purchase #2 $82.47   8.144%   $89.1864   $89.19   $89.20   $89.20  
  Purchase #3 $22.84   13.420%   $25.9051   $25.91   $25.90   $25.90  
  Purchase #4 $45.73   13.720%   $52.0042   $52.00   $52.00   $52.00  
  Purchase #5 $16.32   13.821%   $18.5756   $18.58   $18.60   $18.60  
  Purchase #6 $170.31   7.498%   $183.0798   $183.08   $183.10   $183.10  
  Purchase #7 $221.60   11.685%   $247.494   $247.49   $247.50   $247.50  
  7 purchase totals = $577.88       $637.3196   $637.32   $637.35   $637.40