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 $239.92   6.157%   $254.6919   $254.69   $254.70   $254.70  
  Purchase #2 $215.33   4.477%   $224.9703   $224.97   $224.95   $225.00  
  Purchase #3 $152.58   11.566%   $170.2274   $170.23   $170.25   $170.20  
  Purchase #4 $22.96   13.318%   $26.0178   $26.02   $26.00   $26.00  
  Purchase #5 $243.52   8.577%   $264.4067   $264.41   $264.40   $264.40  
  Purchase #6 $122.98   6.263%   $130.6822   $130.68   $130.70   $130.70  
  Purchase #7 $5.72   8.453%   $6.2035   $6.20   $6.20   $6.20  
  7 purchase totals = $1,003.01       $1,077.1998   $1,077.20   $1,077.20   $1,077.20