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 $244.35   12.522%   $274.9475   $274.95   $274.95   $274.90  
  Purchase #2 $79.33   11.839%   $88.7219   $88.72   $88.70   $88.70  
  Purchase #3 $155.63   7.212%   $166.854   $166.85   $166.85   $166.90  
  Purchase #4 $59.79   10.265%   $65.9274   $65.93   $65.95   $65.90  
  Purchase #5 $147.90   5.696%   $156.3244   $156.32   $156.30   $156.30  
  Purchase #6 $246.93   7.847%   $266.3066   $266.31   $266.30   $266.30  
  Purchase #7 $95.88   7.661%   $103.2254   $103.23   $103.25   $103.20  
  7 purchase totals = $1,029.81       $1,122.3072   $1,122.31   $1,122.30   $1,122.20