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 $243.45   12.279%   $273.3432   $273.34   $273.35   $273.30  
  Purchase #2 $59.57   10.369%   $65.7468   $65.75   $65.75   $65.70  
  Purchase #3 $201.90   8.421%   $218.902   $218.90   $218.90   $218.90  
  Purchase #4 $53.89   9.419%   $58.9659   $58.97   $58.95   $59.00  
  Purchase #5 $208.81   7.132%   $223.7023   $223.70   $223.70   $223.70  
  Purchase #6 $95.72   3.387%   $98.962   $98.96   $98.95   $99.00  
  Purchase #7 $40.62   10.687%   $44.9611   $44.96   $44.95   $45.00  
  7 purchase totals = $903.96       $984.5833   $984.58   $984.55   $984.60