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 $44.95   13.410%   $50.9778   $50.98   $51.00   $51.00  
  Purchase #2 $94.38   11.155%   $104.9081   $104.91   $104.90   $104.90  
  Purchase #3 $23.61   13.110%   $26.7053   $26.71   $26.70   $26.70  
  Purchase #4 $207.71   7.861%   $224.0381   $224.04   $224.05   $224.00  
  Purchase #5 $179.68   3.283%   $185.5789   $185.58   $185.60   $185.60  
  Purchase #6 $87.88   4.704%   $92.0139   $92.01   $92.00   $92.00  
  Purchase #7 $140.80   8.334%   $152.5343   $152.53   $152.55   $152.50  
  7 purchase totals = $779.01       $836.7564   $836.76   $836.80   $836.70