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 $90.51   3.127%   $93.3402   $93.34   $93.35   $93.30  
  Purchase #2 $151.41   9.341%   $165.5532   $165.55   $165.55   $165.60  
  Purchase #3 $68.76   12.628%   $77.443   $77.44   $77.45   $77.40  
  Purchase #4 $31.78   12.532%   $35.7627   $35.76   $35.75   $35.80  
  Purchase #5 $73.75   12.858%   $83.2328   $83.23   $83.25   $83.20  
  Purchase #6 $172.85   8.424%   $187.4109   $187.41   $187.40   $187.40  
  Purchase #7 $56.58   10.177%   $62.3381   $62.34   $62.35   $62.30  
  7 purchase totals = $645.64       $705.0809   $705.07   $705.10   $705.00