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 $151.88   5.866%   $160.7893   $160.79   $160.80   $160.80  
  Purchase #2 $117.70   4.397%   $122.8753   $122.88   $122.90   $122.90  
  Purchase #3 $24.94   3.505%   $25.8141   $25.81   $25.80   $25.80  
  Purchase #4 $137.76   9.665%   $151.0745   $151.07   $151.05   $151.10  
  Purchase #5 $28.59   13.458%   $32.4376   $32.44   $32.45   $32.40  
  Purchase #6 $57.64   7.764%   $62.1152   $62.12   $62.10   $62.10  
  Purchase #7 $79.31   9.490%   $86.8365   $86.84   $86.85   $86.80  
  7 purchase totals = $597.82       $641.9425   $641.95   $641.95   $641.90