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 $170.57   10.117%   $187.8266   $187.83   $187.85   $187.80  
  Purchase #2 $243.64   3.325%   $251.741   $251.74   $251.75   $251.70  
  Purchase #3 $35.57   10.839%   $39.4254   $39.43   $39.45   $39.40  
  Purchase #4 $75.37   12.866%   $85.0671   $85.07   $85.05   $85.10  
  Purchase #5 $125.58   13.165%   $142.1126   $142.11   $142.10   $142.10  
  Purchase #6 $72.34   9.459%   $79.1826   $79.18   $79.20   $79.20  
  Purchase #7 $112.34   12.766%   $126.6813   $126.68   $126.70   $126.70  
  7 purchase totals = $835.41       $912.0366   $912.04   $912.10   $912.00