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 $16.72   8.146%   $18.082   $18.08   $18.10   $18.10  
  Purchase #2 $10.75   10.172%   $11.8435   $11.84   $11.85   $11.80  
  Purchase #3 $118.57   12.327%   $133.1861   $133.19   $133.20   $133.20  
  Purchase #4 $230.78   3.470%   $238.7881   $238.79   $238.80   $238.80  
  Purchase #5 $201.34   9.777%   $221.025   $221.03   $221.05   $221.00  
  Purchase #6 $150.93   10.730%   $167.1248   $167.12   $167.10   $167.10  
  Purchase #7 $180.26   5.134%   $189.5145   $189.51   $189.50   $189.50  
  7 purchase totals = $909.35       $979.5640   $979.56   $979.60   $979.50