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 $16.31   9.623%   $17.8795   $17.88   $17.90   $17.90  
  Purchase #2 $140.28   12.635%   $158.0044   $158.00   $158.00   $158.00  
  Purchase #3 $142.64   4.774%   $149.4496   $149.45   $149.45   $149.40  
  Purchase #4 $141.29   9.823%   $155.1689   $155.17   $155.15   $155.20  
  Purchase #5 $45.59   13.146%   $51.5833   $51.58   $51.60   $51.60  
  Purchase #6 $106.82   8.663%   $116.0738   $116.07   $116.05   $116.10  
  Purchase #7 $238.88   5.518%   $252.0614   $252.06   $252.05   $252.10  
  7 purchase totals = $831.81       $900.2209   $900.21   $900.20   $900.30