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 $235.98   6.365%   $251.0001   $251.00   $251.00   $251.00  
  Purchase #2 $195.60   13.686%   $222.3698   $222.37   $222.35   $222.40  
  Purchase #3 $140.68   10.352%   $155.2432   $155.24   $155.25   $155.20  
  Purchase #4 $71.31   6.717%   $76.0999   $76.10   $76.10   $76.10  
  Purchase #5 $193.28   12.697%   $217.8208   $217.82   $217.80   $217.80  
  Purchase #6 $247.27   11.506%   $275.7209   $275.72   $275.70   $275.70  
  Purchase #7 $182.82   13.230%   $207.0071   $207.01   $207.00   $207.00  
  7 purchase totals = $1,266.94       $1,405.2618   $1,405.26   $1,405.20   $1,405.20