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 $133.57   3.850%   $138.7124   $138.71   $138.70   $138.70  
  Purchase #2 $174.82   3.298%   $180.5856   $180.59   $180.60   $180.60  
  Purchase #3 $85.80   3.864%   $89.1153   $89.12   $89.10   $89.10  
  Purchase #4 $7.44   6.828%   $7.948   $7.95   $7.95   $7.90  
  Purchase #5 $190.46   13.316%   $215.8217   $215.82   $215.80   $215.80  
  Purchase #6 $113.75   5.118%   $119.5717   $119.57   $119.55   $119.60  
  Purchase #7 $202.95   7.865%   $218.912   $218.91   $218.90   $218.90  
  7 purchase totals = $908.79       $970.6667   $970.67   $970.60   $970.60