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 $120.93   7.270%   $129.7216   $129.72   $129.70   $129.70  
  Purchase #2 $100.45   8.200%   $108.6869   $108.69   $108.70   $108.70  
  Purchase #3 $19.76   6.250%   $20.995   $21.00   $21.00   $21.00  
  Purchase #4 $188.50   8.442%   $204.4132   $204.41   $204.40   $204.40  
  Purchase #5 $248.27   3.606%   $257.2226   $257.22   $257.20   $257.20  
  Purchase #6 $31.89   7.515%   $34.2865   $34.29   $34.30   $34.30  
  Purchase #7 $179.95   6.491%   $191.6306   $191.63   $191.65   $191.60  
  7 purchase totals = $889.75       $946.9564   $946.96   $946.95   $946.90