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 $39.90   3.186%   $41.1712   $41.17   $41.15   $41.20  
  Purchase #2 $145.80   13.799%   $165.9189   $165.92   $165.90   $165.90  
  Purchase #3 $184.74   3.193%   $190.6387   $190.64   $190.65   $190.60  
  Purchase #4 $135.87   6.661%   $144.9203   $144.92   $144.90   $144.90  
  Purchase #5 $158.70   9.717%   $174.1209   $174.12   $174.10   $174.10  
  Purchase #6 $108.44   11.775%   $121.2088   $121.21   $121.20   $121.20  
  Purchase #7 $233.86   4.712%   $244.8795   $244.88   $244.90   $244.90  
  7 purchase totals = $1,007.31       $1,082.8583   $1,082.86   $1,082.80   $1,082.80