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 $19.45   6.435%   $20.7016   $20.70   $20.70   $20.70  
  Purchase #2 $140.86   3.229%   $145.4084   $145.41   $145.40   $145.40  
  Purchase #3 $145.61   9.356%   $159.2333   $159.23   $159.25   $159.20  
  Purchase #4 $147.39   3.818%   $153.0174   $153.02   $153.00   $153.00  
  Purchase #5 $106.97   3.675%   $110.9011   $110.90   $110.90   $110.90  
  Purchase #6 $162.55   9.142%   $177.4103   $177.41   $177.40   $177.40  
  Purchase #7 $121.49   10.530%   $134.2829   $134.28   $134.30   $134.30  
  7 purchase totals = $844.32       $900.9550   $900.95   $900.95   $900.90