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 $157.33   6.669%   $167.8223   $167.82   $167.80   $167.80  
  Purchase #2 $71.79   6.737%   $76.6265   $76.63   $76.65   $76.60  
  Purchase #3 $20.90   6.830%   $22.3275   $22.33   $22.35   $22.30  
  Purchase #4 $241.42   10.526%   $266.8319   $266.83   $266.85   $266.80  
  Purchase #5 $206.57   6.155%   $219.2844   $219.28   $219.30   $219.30  
  Purchase #6 $110.52   9.421%   $120.9321   $120.93   $120.95   $120.90  
  Purchase #7 $132.37   11.697%   $147.8533   $147.85   $147.85   $147.90  
  7 purchase totals = $940.90       $1,021.6780   $1,021.67   $1,021.75   $1,021.60