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 $246.74   13.501%   $280.0524   $280.05   $280.05   $280.10  
  Purchase #2 $207.31   6.385%   $220.5467   $220.55   $220.55   $220.50  
  Purchase #3 $196.44   5.734%   $207.7039   $207.70   $207.70   $207.70  
  Purchase #4 $111.68   11.553%   $124.5824   $124.58   $124.60   $124.60  
  Purchase #5 $63.92   6.144%   $67.8472   $67.85   $67.85   $67.80  
  Purchase #6 $125.52   5.136%   $131.9667   $131.97   $131.95   $132.00  
  Purchase #7 $164.90   11.464%   $183.8041   $183.80   $183.80   $183.80  
  7 purchase totals = $1,116.51       $1,216.5034   $1,216.50   $1,216.50   $1,216.50