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 $20.86   11.685%   $23.2975   $23.30   $23.30   $23.30  
  Purchase #2 $219.42   9.345%   $239.9248   $239.92   $239.90   $239.90  
  Purchase #3 $92.44   5.189%   $97.2367   $97.24   $97.25   $97.20  
  Purchase #4 $211.55   5.420%   $223.016   $223.02   $223.00   $223.00  
  Purchase #5 $40.72   10.187%   $44.8681   $44.87   $44.85   $44.90  
  Purchase #6 $55.93   12.217%   $62.763   $62.76   $62.75   $62.80  
  Purchase #7 $81.57   5.816%   $86.3141   $86.31   $86.30   $86.30  
  7 purchase totals = $722.49       $777.4202   $777.42   $777.35   $777.40