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 $129.88   6.307%   $138.0715   $138.07   $138.05   $138.10  
  Purchase #2 $145.49   6.540%   $155.005   $155.01   $155.00   $155.00  
  Purchase #3 $220.99   10.580%   $244.3707   $244.37   $244.35   $244.40  
  Purchase #4 $175.25   4.487%   $183.1135   $183.11   $183.10   $183.10  
  Purchase #5 $137.86   3.287%   $142.3915   $142.39   $142.40   $142.40  
  Purchase #6 $186.48   7.183%   $199.8749   $199.87   $199.85   $199.90  
  Purchase #7 $216.43   12.441%   $243.3561   $243.36   $243.35   $243.40  
  7 purchase totals = $1,212.38       $1,306.1832   $1,306.18   $1,306.10   $1,306.30