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 $127.79   7.668%   $137.5889   $137.59   $137.60   $137.60  
  Purchase #2 $155.29   9.653%   $170.2801   $170.28   $170.30   $170.30  
  Purchase #3 $100.67   13.408%   $114.1678   $114.17   $114.15   $114.20  
  Purchase #4 $5.67   7.581%   $6.0998   $6.10   $6.10   $6.10  
  Purchase #5 $28.43   6.865%   $30.3817   $30.38   $30.40   $30.40  
  Purchase #6 $33.95   9.813%   $37.2815   $37.28   $37.30   $37.30  
  Purchase #7 $135.82   8.761%   $147.7192   $147.72   $147.70   $147.70  
  7 purchase totals = $587.62       $643.5190   $643.52   $643.55   $643.60