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 $79.78   5.441%   $84.1208   $84.12   $84.10   $84.10  
  Purchase #2 $67.51   6.265%   $71.7395   $71.74   $71.75   $71.70  
  Purchase #3 $79.41   9.386%   $86.8634   $86.86   $86.85   $86.90  
  Purchase #4 $26.80   8.129%   $28.9786   $28.98   $29.00   $29.00  
  Purchase #5 $172.64   8.479%   $187.2781   $187.28   $187.30   $187.30  
  Purchase #6 $213.99   13.682%   $243.2681   $243.27   $243.25   $243.30  
  Purchase #7 $168.61   11.106%   $187.3358   $187.34   $187.35   $187.30  
  7 purchase totals = $808.74       $889.5843   $889.59   $889.60   $889.60