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 $63.96   7.743%   $68.9124   $68.91   $68.90   $68.90  
  Purchase #2 $169.87   13.533%   $192.8585   $192.86   $192.85   $192.90  
  Purchase #3 $23.43   8.245%   $25.3618   $25.36   $25.35   $25.40  
  Purchase #4 $119.68   7.848%   $129.0725   $129.07   $129.05   $129.10  
  Purchase #5 $169.84   4.533%   $177.5388   $177.54   $177.55   $177.50  
  Purchase #6 $175.98   3.413%   $181.9862   $181.99   $182.00   $182.00  
  Purchase #7 $55.60   5.122%   $58.4478   $58.45   $58.45   $58.40  
  7 purchase totals = $778.36       $834.1780   $834.18   $834.15   $834.20