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 $30.75   3.264%   $31.7537   $31.75   $31.75   $31.80  
  Purchase #2 $25.84   8.817%   $28.1183   $28.12   $28.10   $28.10  
  Purchase #3 $56.69   3.667%   $58.7688   $58.77   $58.75   $58.80  
  Purchase #4 $86.48   3.139%   $89.1946   $89.19   $89.20   $89.20  
  Purchase #5 $24.32   7.645%   $26.1793   $26.18   $26.20   $26.20  
  Purchase #6 $172.59   8.711%   $187.6243   $187.62   $187.60   $187.60  
  Purchase #7 $48.42   8.493%   $52.5323   $52.53   $52.55   $52.50  
  7 purchase totals = $445.09       $474.1713   $474.16   $474.15   $474.20