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 $48.69   12.790%   $54.9175   $54.92   $54.90   $54.90  
  Purchase #2 $135.97   3.223%   $140.3523   $140.35   $140.35   $140.40  
  Purchase #3 $15.87   8.583%   $17.2321   $17.23   $17.25   $17.20  
  Purchase #4 $172.33   7.361%   $185.0152   $185.02   $185.00   $185.00  
  Purchase #5 $239.95   5.358%   $252.8065   $252.81   $252.80   $252.80  
  Purchase #6 $111.80   8.263%   $121.038   $121.04   $121.05   $121.00  
  Purchase #7 $63.31   10.532%   $69.9778   $69.98   $70.00   $70.00  
  7 purchase totals = $787.92       $841.3394   $841.35   $841.35   $841.30