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 $19.27   8.531%   $20.9139   $20.91   $20.90   $20.90  
  Purchase #2 $22.68   3.265%   $23.4205   $23.42   $23.40   $23.40  
  Purchase #3 $19.82   12.178%   $22.2337   $22.23   $22.25   $22.20  
  Purchase #4 $26.55   10.587%   $29.3608   $29.36   $29.35   $29.40  
  Purchase #5 $97.67   7.582%   $105.0753   $105.08   $105.10   $105.10  
  Purchase #6 $96.84   5.336%   $102.0074   $102.01   $102.00   $102.00  
  Purchase #7 $29.68   11.644%   $33.1359   $33.14   $33.15   $33.10  
  7 purchase totals = $312.51       $336.1475   $336.15   $336.15   $336.10