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 $12.37   7.728%   $13.326   $13.33   $13.35   $13.30  
  Purchase #2 $5.89   4.202%   $6.1375   $6.14   $6.15   $6.10  
  Purchase #3 $27.43   13.612%   $31.1638   $31.16   $31.15   $31.20  
  Purchase #4 $79.27   3.614%   $82.1348   $82.13   $82.15   $82.10  
  Purchase #5 $111.56   9.543%   $122.2062   $122.21   $122.20   $122.20  
  Purchase #6 $57.81   7.252%   $62.0024   $62.00   $62.00   $62.00  
  Purchase #7 $205.99   6.702%   $219.7954   $219.80   $219.80   $219.80  
  7 purchase totals = $500.32       $536.7661   $536.77   $536.80   $536.70