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 $9.99   6.617%   $10.651   $10.65   $10.65   $10.70  
  Purchase #2 $171.86   11.786%   $192.1154   $192.12   $192.10   $192.10  
  Purchase #3 $219.87   4.677%   $230.1533   $230.15   $230.15   $230.20  
  Purchase #4 $185.62   8.723%   $201.8116   $201.81   $201.80   $201.80  
  Purchase #5 $142.74   12.784%   $160.9879   $160.99   $161.00   $161.00  
  Purchase #6 $34.56   7.573%   $37.1772   $37.18   $37.20   $37.20  
  Purchase #7 $148.70   4.545%   $155.4584   $155.46   $155.45   $155.50  
  7 purchase totals = $913.34       $988.3548   $988.36   $988.35   $988.50