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 $236.76   13.377%   $268.4314   $268.43   $268.45   $268.40  
  Purchase #2 $39.78   9.773%   $43.6677   $43.67   $43.65   $43.70  
  Purchase #3 $8.96   3.252%   $9.2514   $9.25   $9.25   $9.30  
  Purchase #4 $41.36   12.617%   $46.5784   $46.58   $46.60   $46.60  
  Purchase #5 $83.81   7.678%   $90.2449   $90.24   $90.25   $90.20  
  Purchase #6 $156.94   10.248%   $173.0232   $173.02   $173.00   $173.00  
  Purchase #7 $160.29   8.445%   $173.8265   $173.83   $173.85   $173.80  
  7 purchase totals = $727.90       $805.0235   $805.02   $805.05   $805.00