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 $18.91   6.213%   $20.0849   $20.08   $20.10   $20.10  
  Purchase #2 $70.42   9.122%   $76.8437   $76.84   $76.85   $76.80  
  Purchase #3 $6.47   13.417%   $7.3381   $7.34   $7.35   $7.30  
  Purchase #4 $139.80   11.760%   $156.2405   $156.24   $156.25   $156.20  
  Purchase #5 $205.60   10.729%   $227.6588   $227.66   $227.65   $227.70  
  Purchase #6 $75.39   3.167%   $77.7776   $77.78   $77.80   $77.80  
  Purchase #7 $7.25   8.248%   $7.848   $7.85   $7.85   $7.80  
  7 purchase totals = $523.84       $573.7916   $573.79   $573.85   $573.70