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 $70.62   13.137%   $79.8973   $79.90   $79.90   $79.90  
  Purchase #2 $183.39   10.714%   $203.0384   $203.04   $203.05   $203.00  
  Purchase #3 $210.34   5.851%   $222.647   $222.65   $222.65   $222.60  
  Purchase #4 $123.64   3.249%   $127.6571   $127.66   $127.65   $127.70  
  Purchase #5 $219.26   3.484%   $226.899   $226.90   $226.90   $226.90  
  Purchase #6 $232.43   9.872%   $255.3755   $255.38   $255.40   $255.40  
  Purchase #7 $128.29   12.850%   $144.7753   $144.78   $144.80   $144.80  
  7 purchase totals = $1,167.97       $1,260.2896   $1,260.31   $1,260.35   $1,260.30