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 $19.83   4.835%   $20.7888   $20.79   $20.80   $20.80  
  Purchase #2 $177.76   7.651%   $191.3604   $191.36   $191.35   $191.40  
  Purchase #3 $216.72   10.187%   $238.7973   $238.80   $238.80   $238.80  
  Purchase #4 $184.42   3.564%   $190.9927   $190.99   $191.00   $191.00  
  Purchase #5 $244.80   5.540%   $258.3619   $258.36   $258.35   $258.40  
  Purchase #6 $158.87   9.390%   $173.7879   $173.79   $173.80   $173.80  
  Purchase #7 $229.91   10.596%   $254.2713   $254.27   $254.25   $254.30  
  7 purchase totals = $1,232.31       $1,328.3603   $1,328.36   $1,328.35   $1,328.50