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 $199.89   13.562%   $226.9991   $227.00   $227.00   $227.00  
  Purchase #2 $23.66   3.443%   $24.4746   $24.47   $24.45   $24.50  
  Purchase #3 $74.59   6.526%   $79.4577   $79.46   $79.45   $79.50  
  Purchase #4 $236.38   12.133%   $265.06   $265.06   $265.05   $265.10  
  Purchase #5 $129.47   6.195%   $137.4907   $137.49   $137.50   $137.50  
  Purchase #6 $99.83   4.162%   $103.9849   $103.98   $104.00   $104.00  
  Purchase #7 $6.80   12.728%   $7.6655   $7.67   $7.65   $7.70  
  7 purchase totals = $770.62       $845.1325   $845.13   $845.10   $845.30