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 $76.29   11.141%   $84.7895   $84.79   $84.80   $84.80  
  Purchase #2 $121.26   9.659%   $132.9725   $132.97   $132.95   $133.00  
  Purchase #3 $69.64   7.519%   $74.8762   $74.88   $74.90   $74.90  
  Purchase #4 $226.25   3.675%   $234.5647   $234.56   $234.55   $234.60  
  Purchase #5 $80.34   3.573%   $83.2105   $83.21   $83.20   $83.20  
  Purchase #6 $169.72   5.248%   $178.6269   $178.63   $178.65   $178.60  
  Purchase #7 $245.37   13.823%   $279.2875   $279.29   $279.30   $279.30  
  7 purchase totals = $988.87       $1,068.3278   $1,068.33   $1,068.35   $1,068.40