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 $29.47   5.152%   $30.9883   $30.99   $31.00   $31.00  
  Purchase #2 $123.40   13.547%   $140.117   $140.12   $140.10   $140.10  
  Purchase #3 $244.79   9.412%   $267.8296   $267.83   $267.85   $267.80  
  Purchase #4 $109.91   3.546%   $113.8074   $113.81   $113.80   $113.80  
  Purchase #5 $243.50   12.829%   $274.7386   $274.74   $274.75   $274.70  
  Purchase #6 $194.72   10.861%   $215.8685   $215.87   $215.85   $215.90  
  Purchase #7 $250.41   6.712%   $267.2175   $267.22   $267.20   $267.20  
  7 purchase totals = $1,196.20       $1,310.5669   $1,310.58   $1,310.55   $1,310.50