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 $29.26   7.514%   $31.4586   $31.46   $31.45   $31.50  
  Purchase #2 $15.93   12.518%   $17.9241   $17.92   $17.90   $17.90  
  Purchase #3 $206.90   11.715%   $231.1383   $231.14   $231.15   $231.10  
  Purchase #4 $147.80   8.449%   $160.2876   $160.29   $160.30   $160.30  
  Purchase #5 $229.73   12.342%   $258.0833   $258.08   $258.10   $258.10  
  Purchase #6 $70.53   12.705%   $79.4908   $79.49   $79.50   $79.50  
  Purchase #7 $186.85   6.255%   $198.5375   $198.54   $198.55   $198.50  
  7 purchase totals = $887.00       $976.9202   $976.92   $976.95   $976.90