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 $80.47   12.830%   $90.7943   $90.79   $90.80   $90.80  
  Purchase #2 $209.67   4.635%   $219.3882   $219.39   $219.40   $219.40  
  Purchase #3 $13.77   8.646%   $14.9606   $14.96   $14.95   $15.00  
  Purchase #4 $13.46   6.127%   $14.2847   $14.28   $14.30   $14.30  
  Purchase #5 $67.45   9.835%   $74.0837   $74.08   $74.10   $74.10  
  Purchase #6 $90.70   10.163%   $99.9178   $99.92   $99.90   $99.90  
  Purchase #7 $130.40   9.620%   $142.9445   $142.94   $142.95   $142.90  
  7 purchase totals = $605.92       $656.3738   $656.36   $656.40   $656.40