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 $16.30   10.261%   $17.9725   $17.97   $17.95   $18.00  
  Purchase #2 $128.60   3.486%   $133.083   $133.08   $133.10   $133.10  
  Purchase #3 $151.60   9.315%   $165.7215   $165.72   $165.70   $165.70  
  Purchase #4 $80.68   8.145%   $87.2514   $87.25   $87.25   $87.30  
  Purchase #5 $80.27   9.386%   $87.8041   $87.80   $87.80   $87.80  
  Purchase #6 $20.50   11.732%   $22.9051   $22.91   $22.90   $22.90  
  Purchase #7 $112.78   8.661%   $122.5479   $122.55   $122.55   $122.50  
  7 purchase totals = $590.73       $637.2855   $637.28   $637.25   $637.30