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 $248.30   11.471%   $276.7825   $276.78   $276.80   $276.80  
  Purchase #2 $23.92   12.397%   $26.8854   $26.89   $26.90   $26.90  
  Purchase #3 $59.83   10.252%   $65.9638   $65.96   $65.95   $66.00  
  Purchase #4 $8.34   12.577%   $9.3889   $9.39   $9.40   $9.40  
  Purchase #5 $31.33   7.501%   $33.6801   $33.68   $33.70   $33.70  
  Purchase #6 $27.57   7.302%   $29.5832   $29.58   $29.60   $29.60  
  Purchase #7 $171.90   3.153%   $177.32   $177.32   $177.30   $177.30  
  7 purchase totals = $571.19       $619.6039   $619.60   $619.65   $619.70