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 $158.80   13.345%   $179.9919   $179.99   $180.00   $180.00  
  Purchase #2 $208.38   4.622%   $218.0113   $218.01   $218.00   $218.00  
  Purchase #3 $189.41   9.855%   $208.0764   $208.08   $208.10   $208.10  
  Purchase #4 $183.60   11.849%   $205.3548   $205.35   $205.35   $205.40  
  Purchase #5 $232.30   8.130%   $251.186   $251.19   $251.20   $251.20  
  Purchase #6 $221.74   10.115%   $244.169   $244.17   $244.15   $244.20  
  Purchase #7 $139.81   7.822%   $150.7459   $150.75   $150.75   $150.70  
  7 purchase totals = $1,334.04       $1,457.5353   $1,457.54   $1,457.55   $1,457.60