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 $54.65   8.496%   $59.2931   $59.29   $59.30   $59.30  
  Purchase #2 $128.94   7.769%   $138.9573   $138.96   $138.95   $139.00  
  Purchase #3 $101.43   11.418%   $113.0113   $113.01   $113.00   $113.00  
  Purchase #4 $192.39   7.658%   $207.1232   $207.12   $207.10   $207.10  
  Purchase #5 $61.54   13.814%   $70.0411   $70.04   $70.05   $70.00  
  Purchase #6 $83.76   4.291%   $87.3541   $87.35   $87.35   $87.40  
  Purchase #7 $123.44   3.183%   $127.3691   $127.37   $127.35   $127.40  
  7 purchase totals = $746.15       $803.1492   $803.14   $803.10   $803.20