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 $132.73   13.226%   $150.2849   $150.28   $150.30   $150.30  
  Purchase #2 $198.79   12.297%   $223.2352   $223.24   $223.25   $223.20  
  Purchase #3 $22.38   11.581%   $24.9718   $24.97   $24.95   $25.00  
  Purchase #4 $206.28   10.771%   $228.4984   $228.50   $228.50   $228.50  
  Purchase #5 $193.67   12.853%   $218.5624   $218.56   $218.55   $218.60  
  Purchase #6 $113.75   8.167%   $123.04   $123.04   $123.05   $123.00  
  Purchase #7 $25.96   6.795%   $27.724   $27.72   $27.70   $27.70  
  7 purchase totals = $893.56       $996.3167   $996.31   $996.30   $996.30