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 $52.45   11.350%   $58.4031   $58.40   $58.40   $58.40  
  Purchase #2 $152.82   12.772%   $172.3382   $172.34   $172.35   $172.30  
  Purchase #3 $172.90   7.644%   $186.1165   $186.12   $186.10   $186.10  
  Purchase #4 $121.43   13.640%   $137.9931   $137.99   $138.00   $138.00  
  Purchase #5 $22.77   7.103%   $24.3874   $24.39   $24.40   $24.40  
  Purchase #6 $228.42   9.680%   $250.5311   $250.53   $250.55   $250.50  
  Purchase #7 $149.68   13.177%   $169.4033   $169.40   $169.40   $169.40  
  7 purchase totals = $900.47       $999.1727   $999.17   $999.20   $999.10