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 $8.35   9.852%   $9.1726   $9.17   $9.15   $9.20  
  Purchase #2 $20.39   7.733%   $21.9668   $21.97   $21.95   $22.00  
  Purchase #3 $199.96   6.841%   $213.6393   $213.64   $213.65   $213.60  
  Purchase #4 $136.55   6.194%   $145.0079   $145.01   $145.00   $145.00  
  Purchase #5 $110.96   12.125%   $124.4139   $124.41   $124.40   $124.40  
  Purchase #6 $82.74   3.324%   $85.4903   $85.49   $85.50   $85.50  
  Purchase #7 $218.36   10.558%   $241.4144   $241.41   $241.40   $241.40  
  7 purchase totals = $777.31       $841.1052   $841.10   $841.05   $841.10