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 $123.87   5.848%   $131.1139   $131.11   $131.10   $131.10  
  Purchase #2 $59.75   3.435%   $61.8024   $61.80   $61.80   $61.80  
  Purchase #3 $151.78   6.323%   $161.377   $161.38   $161.40   $161.40  
  Purchase #4 $39.97   9.611%   $43.8115   $43.81   $43.80   $43.80  
  Purchase #5 $14.45   12.111%   $16.2   $16.20   $16.20   $16.20  
  Purchase #6 $149.98   11.346%   $166.9967   $167.00   $167.00   $167.00  
  Purchase #7 $219.53   6.677%   $234.188   $234.19   $234.20   $234.20  
  7 purchase totals = $759.33       $815.4895   $815.49   $815.50   $815.50