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 $158.59   13.252%   $179.6063   $179.61   $179.60   $179.60  
  Purchase #2 $124.87   12.399%   $140.3526   $140.35   $140.35   $140.40  
  Purchase #3 $57.74   9.598%   $63.2819   $63.28   $63.30   $63.30  
  Purchase #4 $227.53   11.139%   $252.8746   $252.87   $252.85   $252.90  
  Purchase #5 $191.55   4.562%   $200.2885   $200.29   $200.30   $200.30  
  Purchase #6 $117.61   7.588%   $126.5342   $126.53   $126.55   $126.50  
  Purchase #7 $35.68   9.407%   $39.0364   $39.04   $39.05   $39.00  
  7 purchase totals = $913.57       $1,001.9745   $1,001.97   $1,002.00   $1,002.00