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 $133.31   3.599%   $138.1078   $138.11   $138.10   $138.10  
  Purchase #2 $172.33   5.111%   $181.1378   $181.14   $181.15   $181.10  
  Purchase #3 $208.90   5.805%   $221.0266   $221.03   $221.05   $221.00  
  Purchase #4 $155.42   13.129%   $175.8251   $175.83   $175.85   $175.80  
  Purchase #5 $132.30   5.795%   $139.9668   $139.97   $139.95   $140.00  
  Purchase #6 $250.56   5.843%   $265.2002   $265.20   $265.20   $265.20  
  Purchase #7 $90.27   7.318%   $96.876   $96.88   $96.90   $96.90  
  7 purchase totals = $1,143.09       $1,218.1403   $1,218.16   $1,218.20   $1,218.10