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 $7.40   5.272%   $7.7901   $7.79   $7.80   $7.80  
  Purchase #2 $55.40   9.805%   $60.832   $60.83   $60.85   $60.80  
  Purchase #3 $52.94   12.718%   $59.6729   $59.67   $59.65   $59.70  
  Purchase #4 $8.73   8.365%   $9.4603   $9.46   $9.45   $9.50  
  Purchase #5 $75.31   10.709%   $83.3749   $83.37   $83.35   $83.40  
  Purchase #6 $30.95   10.128%   $34.0846   $34.08   $34.10   $34.10  
  Purchase #7 $23.57   13.518%   $26.7562   $26.76   $26.75   $26.80  
  7 purchase totals = $254.30       $281.9710   $281.96   $281.95   $282.10