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 $34.94   12.605%   $39.3442   $39.34   $39.35   $39.30  
  Purchase #2 $90.40   10.809%   $100.1713   $100.17   $100.15   $100.20  
  Purchase #3 $186.87   11.812%   $208.9431   $208.94   $208.95   $208.90  
  Purchase #4 $218.54   3.342%   $225.8436   $225.84   $225.85   $225.80  
  Purchase #5 $46.26   9.872%   $50.8268   $50.83   $50.85   $50.80  
  Purchase #6 $155.55   11.483%   $173.4118   $173.41   $173.40   $173.40  
  Purchase #7 $104.29   13.121%   $117.9739   $117.97   $117.95   $118.00  
  7 purchase totals = $836.85       $916.5147   $916.50   $916.50   $916.40