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 $193.51   12.782%   $218.2444   $218.24   $218.25   $218.20  
  Purchase #2 $240.88   7.673%   $259.3627   $259.36   $259.35   $259.40  
  Purchase #3 $136.28   13.643%   $154.8727   $154.87   $154.85   $154.90  
  Purchase #4 $213.46   13.640%   $242.5759   $242.58   $242.60   $242.60  
  Purchase #5 $186.72   9.846%   $205.1045   $205.10   $205.10   $205.10  
  Purchase #6 $188.31   9.205%   $205.6439   $205.64   $205.65   $205.60  
  Purchase #7 $54.33   9.352%   $59.4109   $59.41   $59.40   $59.40  
  7 purchase totals = $1,213.49       $1,345.2150   $1,345.20   $1,345.20   $1,345.20