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 $81.58   3.221%   $84.2077   $84.21   $84.20   $84.20  
  Purchase #2 $189.99   8.409%   $205.9663   $205.97   $205.95   $206.00  
  Purchase #3 $6.66   13.500%   $7.5591   $7.56   $7.55   $7.60  
  Purchase #4 $200.35   6.649%   $213.6713   $213.67   $213.65   $213.70  
  Purchase #5 $233.33   11.609%   $260.4173   $260.42   $260.40   $260.40  
  Purchase #6 $236.98   13.190%   $268.2377   $268.24   $268.25   $268.20  
  Purchase #7 $196.81   6.812%   $210.2167   $210.22   $210.20   $210.20  
  7 purchase totals = $1,145.70       $1,250.2761   $1,250.29   $1,250.20   $1,250.30