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 $242.99   12.792%   $274.0733   $274.07   $274.05   $274.10  
  Purchase #2 $157.28   10.293%   $173.4688   $173.47   $173.45   $173.50  
  Purchase #3 $80.45   8.526%   $87.3092   $87.31   $87.30   $87.30  
  Purchase #4 $236.27   6.651%   $251.9843   $251.98   $252.00   $252.00  
  Purchase #5 $102.59   9.666%   $112.5063   $112.51   $112.50   $112.50  
  Purchase #6 $168.95   6.132%   $179.31   $179.31   $179.30   $179.30  
  Purchase #7 $17.81   13.790%   $20.266   $20.27   $20.25   $20.30  
  7 purchase totals = $1,006.34       $1,098.9179   $1,098.92   $1,098.85   $1,099.00