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 $37.55   4.186%   $39.1218   $39.12   $39.10   $39.10  
  Purchase #2 $172.77   4.285%   $180.1732   $180.17   $180.15   $180.20  
  Purchase #3 $128.31   4.182%   $133.6759   $133.68   $133.70   $133.70  
  Purchase #4 $32.32   9.284%   $35.3206   $35.32   $35.30   $35.30  
  Purchase #5 $20.65   4.108%   $21.4983   $21.50   $21.50   $21.50  
  Purchase #6 $37.47   4.613%   $39.1985   $39.20   $39.20   $39.20  
  Purchase #7 $182.48   6.420%   $194.1952   $194.20   $194.20   $194.20  
  7 purchase totals = $611.55       $643.1835   $643.19   $643.15   $643.20