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 $129.25   3.472%   $133.7376   $133.74   $133.75   $133.70  
  Purchase #2 $7.61   7.845%   $8.207   $8.21   $8.20   $8.20  
  Purchase #3 $91.62   12.730%   $103.2832   $103.28   $103.30   $103.30  
  Purchase #4 $11.41   6.222%   $12.1199   $12.12   $12.10   $12.10  
  Purchase #5 $20.34   8.461%   $22.061   $22.06   $22.05   $22.10  
  Purchase #6 $147.50   11.277%   $164.1336   $164.13   $164.15   $164.10  
  Purchase #7 $105.65   5.591%   $111.5569   $111.56   $111.55   $111.60  
  7 purchase totals = $513.38       $555.0992   $555.10   $555.10   $555.10