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 $204.33   10.122%   $225.0123   $225.01   $225.00   $225.00  
  Purchase #2 $85.31   11.173%   $94.8417   $94.84   $94.85   $94.80  
  Purchase #3 $229.67   13.731%   $261.206   $261.21   $261.20   $261.20  
  Purchase #4 $63.33   9.656%   $69.4451   $69.45   $69.45   $69.40  
  Purchase #5 $23.74   12.461%   $26.6982   $26.70   $26.70   $26.70  
  Purchase #6 $57.74   9.226%   $63.0671   $63.07   $63.05   $63.10  
  Purchase #7 $240.51   7.552%   $258.6733   $258.67   $258.65   $258.70  
  7 purchase totals = $904.63       $998.9437   $998.95   $998.90   $998.90