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 $12.78   8.615%   $13.881   $13.88   $13.90   $13.90  
  Purchase #2 $114.52   5.865%   $121.2366   $121.24   $121.25   $121.20  
  Purchase #3 $5.91   7.368%   $6.3454   $6.35   $6.35   $6.30  
  Purchase #4 $128.62   11.245%   $143.0833   $143.08   $143.10   $143.10  
  Purchase #5 $162.25   11.393%   $180.7351   $180.74   $180.75   $180.70  
  Purchase #6 $209.75   6.309%   $222.9831   $222.98   $223.00   $223.00  
  Purchase #7 $102.78   10.702%   $113.7795   $113.78   $113.80   $113.80  
  7 purchase totals = $736.61       $802.0440   $802.05   $802.15   $802.00