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 $167.51   6.164%   $177.8353   $177.84   $177.85   $177.80  
  Purchase #2 $113.25   8.118%   $122.4436   $122.44   $122.45   $122.40  
  Purchase #3 $28.43   4.835%   $29.8046   $29.80   $29.80   $29.80  
  Purchase #4 $51.40   3.300%   $53.0962   $53.10   $53.10   $53.10  
  Purchase #5 $37.57   11.228%   $41.7884   $41.79   $41.80   $41.80  
  Purchase #6 $158.99   8.828%   $173.0256   $173.03   $173.05   $173.00  
  Purchase #7 $40.96   4.354%   $42.7434   $42.74   $42.75   $42.70  
  7 purchase totals = $598.11       $640.7371   $640.74   $640.80   $640.60