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 $79.49   12.243%   $89.222   $89.22   $89.20   $89.20  
  Purchase #2 $99.93   5.654%   $105.58   $105.58   $105.60   $105.60  
  Purchase #3 $55.49   4.249%   $57.8478   $57.85   $57.85   $57.80  
  Purchase #4 $110.32   3.124%   $113.7664   $113.77   $113.75   $113.80  
  Purchase #5 $19.51   7.606%   $20.9939   $20.99   $21.00   $21.00  
  Purchase #6 $18.46   13.301%   $20.9154   $20.92   $20.90   $20.90  
  Purchase #7 $26.97   10.453%   $29.7892   $29.79   $29.80   $29.80  
  7 purchase totals = $410.17       $438.1147   $438.12   $438.10   $438.10