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 $6.42   12.237%   $7.2056   $7.21   $7.20   $7.20  
  Purchase #2 $153.49   5.631%   $162.133   $162.13   $162.15   $162.10  
  Purchase #3 $149.69   10.334%   $165.159   $165.16   $165.15   $165.20  
  Purchase #4 $185.90   9.332%   $203.2482   $203.25   $203.25   $203.20  
  Purchase #5 $29.74   6.647%   $31.7168   $31.72   $31.70   $31.70  
  Purchase #6 $170.26   6.619%   $181.5295   $181.53   $181.55   $181.50  
  Purchase #7 $153.27   8.326%   $166.0313   $166.03   $166.05   $166.00  
  7 purchase totals = $848.77       $917.0234   $917.03   $917.05   $916.90