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 $245.81   12.172%   $275.73   $275.73   $275.75   $275.70  
  Purchase #2 $155.68   12.644%   $175.3642   $175.36   $175.35   $175.40  
  Purchase #3 $167.34   8.119%   $180.9263   $180.93   $180.95   $180.90  
  Purchase #4 $229.51   12.818%   $258.9286   $258.93   $258.95   $258.90  
  Purchase #5 $93.46   10.744%   $103.5013   $103.50   $103.50   $103.50  
  Purchase #6 $227.65   9.847%   $250.0667   $250.07   $250.05   $250.10  
  Purchase #7 $71.49   3.566%   $74.0393   $74.04   $74.05   $74.00  
  7 purchase totals = $1,190.94       $1,318.5564   $1,318.56   $1,318.60   $1,318.50