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 $38.75   10.765%   $42.9214   $42.92   $42.90   $42.90  
  Purchase #2 $243.56   11.775%   $272.2392   $272.24   $272.25   $272.20  
  Purchase #3 $191.86   9.406%   $209.9064   $209.91   $209.90   $209.90  
  Purchase #4 $221.57   4.266%   $231.0222   $231.02   $231.00   $231.00  
  Purchase #5 $159.68   10.241%   $176.0328   $176.03   $176.05   $176.00  
  Purchase #6 $178.82   5.737%   $189.0789   $189.08   $189.10   $189.10  
  Purchase #7 $108.30   9.822%   $118.9372   $118.94   $118.95   $118.90  
  7 purchase totals = $1,142.54       $1,240.1381   $1,240.14   $1,240.15   $1,240.00