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 $232.43   12.153%   $260.6772   $260.68   $260.70   $260.70  
  Purchase #2 $32.80   10.782%   $36.3365   $36.34   $36.35   $36.30  
  Purchase #3 $52.25   9.829%   $57.3857   $57.39   $57.40   $57.40  
  Purchase #4 $29.48   13.837%   $33.5591   $33.56   $33.55   $33.60  
  Purchase #5 $117.26   11.670%   $130.9442   $130.94   $130.95   $130.90  
  Purchase #6 $213.75   12.542%   $240.5585   $240.56   $240.55   $240.60  
  Purchase #7 $96.83   10.219%   $106.7251   $106.73   $106.75   $106.70  
  7 purchase totals = $774.80       $866.1863   $866.20   $866.25   $866.20