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 $17.69   13.606%   $20.0969   $20.10   $20.10   $20.10  
  Purchase #2 $250.87   11.273%   $279.1506   $279.15   $279.15   $279.20  
  Purchase #3 $204.82   12.288%   $229.9883   $229.99   $230.00   $230.00  
  Purchase #4 $91.40   9.211%   $99.8189   $99.82   $99.80   $99.80  
  Purchase #5 $59.86   6.540%   $63.7748   $63.77   $63.75   $63.80  
  Purchase #6 $28.42   5.661%   $30.0289   $30.03   $30.05   $30.00  
  Purchase #7 $207.55   7.326%   $222.7551   $222.76   $222.75   $222.80  
  7 purchase totals = $860.61       $945.6135   $945.62   $945.60   $945.70