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 $170.28   4.462%   $177.8779   $177.88   $177.90   $177.90  
  Purchase #2 $50.60   3.375%   $52.3078   $52.31   $52.30   $52.30  
  Purchase #3 $150.53   3.159%   $155.2852   $155.29   $155.30   $155.30  
  Purchase #4 $126.83   6.804%   $135.4595   $135.46   $135.45   $135.50  
  Purchase #5 $102.39   9.305%   $111.9174   $111.92   $111.90   $111.90  
  Purchase #6 $33.60   6.294%   $35.7148   $35.71   $35.70   $35.70  
  Purchase #7 $15.92   12.311%   $17.8799   $17.88   $17.90   $17.90  
  7 purchase totals = $650.15       $686.4425   $686.45   $686.45   $686.50